nuttx: Provide a default WASM_STACK_GUARD_SIZE (#1386)

Unbreak builds with a bit older nuttx/apps for now.

Note: The default value here (0) was chosen to match the new default
in nuttx/apps, which is different from the old value used before
the introduction of WASM_STACK_GUARD_SIZE. (1024)

product-mini/platforms/nuttx/wamr.mk

@@ -147,7 +147,11 @@ CSRCS += wasm_loader.c
 endif
 endif
 
+ifeq ($(CONFIG_INTERPRETERS_WAMR_STACK_GUARD_SIZE),)
+CFLAGS += -DWASM_STACK_GUARD_SIZE=0
+else
 CFLAGS += -DWASM_STACK_GUARD_SIZE=CONFIG_INTERPRETERS_WAMR_STACK_GUARD_SIZE
+endif
 
 ifeq ($(CONFIG_INTERPRETERS_WAMR_SHARED_MEMORY),y)
 CFLAGS += -DWASM_ENABLE_SHARED_MEMORY=1