Pacify GCC 9 -Wredundant-decls

* src/gmalloc.c (_fraghead) [!HYBRID_MALLOC]:
Make it static in this case, too.  This avoids having both
‘extern struct list _fraghead[];’ and
‘static struct list _fraghead[BLOCKLOG];’, which
GCC 9 complains about.

src/gmalloc.c

@@ -182,7 +182,7 @@ struct list
   };
 
 /* Free list headers for each fragment size.  */
-extern struct list _fraghead[];
+static struct list _fraghead[BLOCKLOG];
 
 /* List of blocks allocated with aligned_alloc and friends.  */
 struct alignlist
@@ -339,9 +339,6 @@ size_t _heapindex;
 /* Limit of valid info table indices.  */
 size_t _heaplimit;
 
-/* Free lists for each fragment size.  */
-struct list _fraghead[BLOCKLOG];
-
 /* Instrumentation.  */
 size_t _chunks_used;
 size_t _bytes_used;
@@ -351,10 +348,6 @@ size_t _bytes_free;
 /* Are you experienced?  */
 int __malloc_initialized;
 
-#else
-
-static struct list _fraghead[BLOCKLOG];
-
 #endif /* HYBRID_MALLOC */
 
 /* Number of extra blocks to get each time we ask for more core.