usage: add NORETURN to BUG() function definitions

Commit d819374 ("usage.c: add BUG() function", 12-05-2017) added the
BUG() functions and macros as a replacement for calls to die("BUG: ..").
The use of NORETURN on the declarations (in git-compat-util.h) and the
lack of NORETURN on the function definitions, however, leads sparse to
complain thus:

      SP usage.c
  usage.c:220:6: error: symbol 'BUG_fl' redeclared with different type
  (originally declared at git-compat-util.h:1074) - different modifiers

In order to suppress the sparse error, add the NORETURN to the function
definitions.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

usage.c

@@ -217,15 +217,15 @@ static NORETURN void BUG_vfl(const char *file, int line, const char *fmt, va_lis
 }
 
 #ifdef HAVE_VARIADIC_MACROS
-void BUG_fl(const char *file, int line, const char *fmt, ...)
+NORETURN void BUG_fl(const char *file, int line, const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);
 	BUG_vfl(file, line, fmt, ap);
 	va_end(ap);
 }
 #else
-void BUG(const char *fmt, ...)
+NORETURN void BUG(const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);