Forward declare mallocx in isc/mem.h

cmocka.h and jemalloc.h/malloc_np.h has conflicting macro definitions.
While fixing them with push_macro for only malloc is done below, we only
need the non-standard mallocx interface which is easy to just define by
ourselves.

(cherry picked from commit 197de93bdc)

lib/isc/include/isc/mem.h

@@ -190,7 +190,13 @@ extern unsigned int isc_mem_defaultflags;
  * for more information.
  */
 #if HAVE_JEMALLOC
-#include <jemalloc/jemalloc.h>
+
+/*
+ * cmocka.h has confliction definitions with the jemalloc header but we only
+ * need the mallocx symbol from jemalloc.
+ */
+void *
+mallocx(size_t size, int flags);
 
 extern volatile void *isc__mem_malloc;