Dummy size for heap to avoid GCC 8/9 warnings

Make GCC think _heap_start is large enough to avoid the basic (and
incorrect) bounds-checking warnings it produces.  The size chosen is
arbitrary and does not affect the actual size of the heap in any way.

Author: earlephilhower

cores/esp8266/umm_malloc/umm_malloc_cfg.h

@@ -91,7 +91,7 @@ void* realloc_loc (void* p, size_t s, const char* file, int line);
  #define UMM_BEST_FIT
 
 /* Start addresses and the size of the heap */
-extern char _heap_start;
+extern char _heap_start[1024];
 #define UMM_MALLOC_CFG__HEAP_ADDR   ((uint32_t)&_heap_start)
 #define UMM_MALLOC_CFG__HEAP_SIZE   ((size_t)(0x3fffc000 - UMM_MALLOC_CFG__HEAP_ADDR))