Move XMALLOC_MAGIC definition into xmalloc.c.

Private, does not need to be exposed externally.

src/common/xmalloc.c

@@ -63,6 +63,7 @@ static void malloc_assert_failed(char *, const char *, int,
           } while (0)
 #endif /* NDEBUG */
 
+#define XMALLOC_MAGIC 0x42
 
 /*
  * "Safe" version of malloc().

src/common/xmalloc.h

@@ -97,6 +97,4 @@ void slurm_xfree(void **, const char *, int, const char *);
 void *slurm_xrealloc(void **, size_t, bool, const char *, int, const char *);
 size_t slurm_xsize(void *, const char *, int, const char *);
 
-#define XMALLOC_MAGIC 0x42
-
 #endif /* !_XMALLOC_H */