alias basename to glibc name for it, to meet abi goals

note that regardless of the name used, basename is always conformant.
it never takes on the bogus gnu behavior, unlike glibc where basename
is nonconformant when declared manually without including libgen.h.

src/misc/basename.c

 #include <string.h>
 #include <libgen.h>
+#include "libc.h"
 
 char *basename(char *s)
 {
@@ -10,3 +11,5 @@ char *basename(char *s)
 	for (; i&&s[i-1]!='/'; i--);
 	return s+i;
 }
+
+weak_alias(basename, __xpg_basename);