Fix cross-builds broken from C++-creep

[Hans-Peter Nilsson]

All cross-builds are "still" done as C.  In C++ you don't need
the missing struct qualifier or the typedef in "typedef struct
gfc_expr ... gfc_expr;" (the struct declaration suffices) as
there's no separate struct namespace IIUC.

Doesn't this show a bug in the compatibility warning system, or
is that turned off when bootstrapping as C++?

Anyway, committed as obvious after a cris-elf build has passed
the point of failure, which looked as follows (first five lines
of errors).

...
gcc -c  -DIN_GCC_FRONTEND -g -O2 -DIN_GCC -DCROSS_DIRECTORY_STRUCTURE  -W -Wall -Wwrite-strings -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Wold-style-definition -Wc++-compat -fno-common  -DHAVE_CONFIG_H -I. -Ifortran -I/tmp/fail0102-96/gcc/gcc -I/tmp/fail0102-96/gcc/gcc/fortran -I/tmp/fail0102-96/gcc/gcc/../include -I/tmp/fail0102-96/gcc/gcc/../libcpp/include -I/tmp/fail0102-96/gccobj/./gmp -I/tmp/fail0102-96/gcc/gmp -I/tmp/fail0102-96/gccobj/./mpfr -I/tmp/fail0102-96/gcc/mpfr -I/tmp/fail0102-96/gcc/mpc/src  -I/tmp/fail0102-96/gcc/gcc/../libdecnumber -I/tmp/fail0102-96/gcc/gcc/../libdecnumber/dpd -I../libdecnumber    /tmp/fail0102-96/gcc/gcc/fortran/arith.c -o fortran/arith.o
In file included from /tmp/fail0102-96/gcc/gcc/fortran/arith.c:31:
/tmp/fail0102-96/gcc/gcc/fortran/gfortran.h:1702: error: expected specifier-qualifier-list before 'gfc_expr'
/tmp/fail0102-96/gcc/gcc/fortran/arith.c: In function 'gfc_arith_not':
/tmp/fail0102-96/gcc/gcc/fortran/arith.c:418: error: 'gfc_expr' has no member named 'value'
/tmp/fail0102-96/gcc/gcc/fortran/arith.c:418: error: 'gfc_expr' has no member named 'value'
/tmp/fail0102-96/gcc/gcc/fortran/arith.c: In function 'gfc_arith_and':
/tmp/fail0102-96/gcc/gcc/fortran/arith.c:432: error: 'gfc_expr' has no member named 'value'
/tmp/fail0102-96/gcc/gcc/fortran/arith.c:432: error: 'gfc_expr' has no member named 'value'

fortran:
	* gfortran.h (struct gfc_expr): Add missing "struct"
	qualifier for member base_expr.

Index: gcc/fortran/gfortran.h
===================================================================
--- gcc/fortran/gfortran.h	(revision 182825)
+++ gcc/fortran/gfortran.h	(working copy)
@@ -1699,7 +1699,7 @@ typedef struct gfc_expr
 
   /* Used to store the base expression in component calls, when the expression
      is not a variable.  */
-  gfc_expr *base_expr;
+  struct gfc_expr *base_expr;
 
   /* is_boz is true if the integer is regarded as BOZ bitpatten and is_snan
      denotes a signalling not-a-number.  */

brgds, H-P