Fix int to pointer conversion for clang

Note - this parameter was not actually used inside eval_buffer because of a define.

Bug: https://bugs.gentoo.org/875449

--- a/src/kmk/loadapi.c
+++ b/src/kmk/loadapi.c
@@ -20,6 +20,8 @@
 #include "variable.h"
 #include "dep.h"
 
+#include <string.h>
+
 /* Allocate a buffer in our context, so we can free it.  */
 char *
 gmk_alloc (unsigned int len)