Use noreturn attribute for printexit

This tells the compiler and static analyzers that we exit; in
this case clang-analyzer was warning about a possible `NULL` deref
in `mountcomposefs.c` that wasn't reachable.

That said, this function is the same thing as `errx`, but we can
fix that as a followup.

Signed-off-by: Colin Walters <walters@verbum.org>

tools/cfs-fuse.c

@@ -56,6 +56,7 @@ uint64_t erofs_build_time;
 uint32_t erofs_build_time_nsec;
 int basedir_fd;
 
+static void printexit(const char *format, ...) __attribute__((noreturn));
 static void printexit(const char *format, ...)
 {
 	va_list args;

tools/mountcomposefs.c

@@ -37,6 +37,7 @@
 
 #include "libcomposefs/lcfs-mount.h"
 
+static void printexit(const char *format, ...) __attribute__((noreturn));
 static void printexit(const char *format, ...)
 {
 	va_list args;