fix: use ifdef for NNG_HAVE_BACKTRACE

In some places, we use ifdef, and others if.

This normalizes for always using ifdef, so we can compile when
this macro is not defined.

src/core/panic.c

@@ -23,7 +23,7 @@
 void
 nni_show_backtrace(void)
 {
-#if NNG_HAVE_BACKTRACE
+#ifdef NNG_HAVE_BACKTRACE
 	void *frames[50];
 	int   nframes;