Revert "Disable -Wmissing-prototypes for internal linkage functions that...

Revert "Disable -Wmissing-prototypes for internal linkage functions that aren't explicitly marked "static""

Regresses:
typedef struct {
  static void  f() {
  }
} a_t;

Causing this to error instead of warn, because the linkage is computed
earlier/too early perhaps. I'll send out a review to see if there's some
other path forward or if this is an acceptable regression, etc.

This reverts commit 275c5622.