[PATCH] bpf: Fix unused variable warning

[Jose E. Marchesi]

Hi Jan.

> Building for for bpf-unknown-none target, recent GCCs will issue an unused
> variable warning as the REGNO_REG_CLASS macro doesn't actually use its
> argument. Reference the argument as (void) to silence the warning.
>
> .../gcc/configure --prefix=... --enable-werror-always --enable-languages=all --disable-gcov --disable-shared --disable-threads --target=bpf-unknown-none --without-headers
> [...]
> make V=1 all-gcc
> [...]
>
> /usr/lib/gcc-snapshot/bin/g++  -fno-PIE -c   -g -O2   -DIN_GCC  -DCROSS_DIRECTORY_STRUCTURE   -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc/gcc -I../../gcc/gcc/. -I../../gcc/gcc/../include -I../../gcc/gcc/../libcpp/include -I../../gcc/gcc/../libcody  -I../../gcc/gcc/../libdecnumber -I../../gcc/gcc/../libdecnumber/dpd -I../libdecnumber -I../../gcc/gcc/../libbacktrace   -o regcprop.o -MT regcprop.o -MMD -MP -MF ./.deps/regcprop.TPo ../../gcc/gcc/regcprop.cc
> ../../gcc/gcc/regcprop.cc: In function 'bool copyprop_hardreg_forward_1(basic_block, value_data*)':
> ../../gcc/gcc/regcprop.cc:794:24: error: unused variable 'regno' [-Werror=unused-variable]
>   794 |           unsigned int regno = REGNO (SET_SRC (set));
>       |                        ^~~~~
> cc1plus: all warnings being treated as errors
> make[1]: *** [Makefile:1146: regcprop.o] Error 1
> make[1]: Leaving directory '/var/lib/laminar/run/gcc-bpf-unknown-none/1/toolchain-build/gcc'
> make: *** [Makefile:4565: all-gcc] Error 2
>
>
> Ok for HEAD?

OK.  Thanks.