fundamental: Move declaration of free() to macro-fundamental.h

Let's put it together with the macro that needs it (mfree()). Also,
get rid of the unnecessary <stdlib.h> include in iovec-util-fundamental.h.

src/fundamental/iovec-util-fundamental.h

@@ -2,7 +2,6 @@
 #pragma once
 
 #if !SD_BOOT
-#include <stdlib.h>
 #include <sys/uio.h>
 #endif
 
@@ -15,10 +14,6 @@ struct iovec {
         void *iov_base;
         size_t iov_len;
 };
-
-DISABLE_WARNING_REDUNDANT_DECLS;
-void free(void *p);
-REENABLE_WARNING;
 #endif
 
 /* This accepts both const and non-const pointers */

src/fundamental/macro-fundamental.h

@@ -396,6 +396,10 @@ assert_cc(sizeof(long long) == sizeof(intmax_t));
  */
 #define STRLEN(x) (sizeof(""x"") - sizeof(typeof(x[0])))
 
+DISABLE_WARNING_REDUNDANT_DECLS;
+void free(void *p);
+REENABLE_WARNING;
+
 #define mfree(memory)                           \
         ({                                      \
                 free(memory);                   \