lto-plugin.c: c23 compatibility: char * -> const char *

lto-plugin.c: c23 compatibility: char * -> const char *

There are two uses of strrchr with const char * parameters in
lto-plugin.c where the result gets assigned to a non-const char *
variable.

Make the variable const char * so that we don't throw away const-ness.
I've seen clang produce warnings about this when compiling lto-plugin.c.

Regtested and bootstrapped on x86_64 linux.

lto-plugin/ChangeLog:

	* lto-plugin.c (process_symtab): char *s -> const char *s.
	(process_symtab_extension): Ditto.

Signed-off-by: Filip Kastl <fkastl@suse.cz>