gthr-gnat.[ch] introduce new warnings

[Zack Weinberg]

gthr-gnat.c:40: warning: no previous prototype for `__gnat_default_lock'
gthr-gnat.c:46: warning: no previous prototype for `__gnat_default_unlock'
gthr-gnat.c:50: warning: function declaration isn't a prototype
gthr-gnat.c:51: warning: function declaration isn't a prototype
gthr-gnat.c:55: warning: function declaration isn't a prototype
gthr-gnat.c:56: warning: function declaration isn't a prototype
gthr-gnat.h:37: warning: function declaration isn't a prototype

Please fix.

zw