Fix the reallocarray header by providing a prototype.

include/extern/reallocarray.h

@@ -1,10 +1,14 @@
 #ifndef EXTERN_REALLOCARRAY_H
 #define EXTERN_REALLOCARRAY_H
 
+#ifdef REALLOCARRAY_IN_LIBC
 #include <stdlib.h>
+#else
 
-#ifndef REALLOCARRAY_IN_LIBC
 #define reallocarray rgbds_reallocarray
+
+void *reallocarray(void *, size_t, size_t);
+
 #endif
 
 #endif