memblock test: fix implicit declaration of function 'memparse'

Commit 1e4c64b71c ("mm/memblock: Add "reserve_mem" to reserved named
memory at boot up") introduce the usage of memparse(), which is not
defined in memblock test.

Add the definition and link it to fix the build.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Link: https://lore.kernel.org/r/20240806010319.29194-3-richard.weiyang@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

tools/include/linux/string.h

@@ -47,4 +47,5 @@ extern char * __must_check skip_spaces(const char *);
 extern char *strim(char *);
 
 extern void *memchr_inv(const void *start, int c, size_t bytes);
+extern unsigned long long memparse(const char *ptr, char **retptr);
 #endif /* _TOOLS_LINUX_STRING_H_ */