Android NDK: no index function in strings.h

due to Android NDK's strings.h doesn't have index function.
Declare this function in CRIU, just like povit_root.

still need to provide index function implement, for link CRIU.

Cc: Chen Hu <hu1.chen@intel.com>
Signed-off-by: Zhang Ning <ning.a.zhang@intel.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>

criu/config.c

@@ -378,6 +378,8 @@ Esyntax:
 	return -1;
 }
 
+extern char *index(const char *s, int c);
+
 static size_t parse_size(char *optarg)
 {
 	if (index(optarg, 'K'))