Make heap an undefined extend array

Instead of a bogus size, use an indefinite size for the heap to avoid
GCC warnings

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[1024];
+extern char _heap_start[];
 #define UMM_MALLOC_CFG__HEAP_ADDR   ((uint32_t)&_heap_start)
 #define UMM_MALLOC_CFG__HEAP_SIZE   ((size_t)(0x3fffc000 - UMM_MALLOC_CFG__HEAP_ADDR))