Enable a few additional warnings for CI and fix the issues they uncovered

- `-Wmissing-prototypes` and `-Wmissing-variable-declarations` are
  helpful for finding dead code and/or things that should be `static`
  but aren't marked as such.
- `-Wunused-macros` is of similar utility, but for identifying dead macros.
- `-Wunreachable-code` and `-Wunreachable-code-aggressive` do exactly
  what they say: flag unreachable code.

src/inspect.c

@@ -1,5 +1,6 @@
 #include "jemalloc/internal/jemalloc_preamble.h"
 #include "jemalloc/internal/jemalloc_internal_includes.h"
+#include "jemalloc/internal/inspect.h"
 
 void
 inspect_extent_util_stats_get(tsdn_t *tsdn, const void *ptr, size_t *nfree,