mprotect(): Change prototype to comply to POSIX.

Our mprotect() function seems to take a "const void *" address to the
pages whose permissions need to be adjusted. POSIX uses "void *". Simply
stick to the POSIX one to prevent us from writing unportable code.

PR:		211423 (exp-run)
Tested by:	antoine@ (Thanks!)

newlib/libc/sys/rtems/include/sys/mman.h

@@ -258,7 +258,7 @@ int	mlock(const void *, size_t);
 #define	_MMAP_DECLARED
 void *	mmap(void *, size_t, int, int, int, off_t);
 #endif
-int	mprotect(const void *, size_t, int);
+int	mprotect(void *, size_t, int);
 int	msync(void *, size_t, int);
 int	munlock(const void *, size_t);
 int	munmap(void *, size_t);