git-compat-util.h: Honor HP C's noreturn attribute

git-compat-util.h: Honor HP C's noreturn attribute

HP C for Integrity servers (Itanium) gained support for noreturn
attribute sometime in 2006. It was released in Compiler Version
A.06.10 and made available in July 2006.

The __HP_cc define detects the HP C compiler version.  Precede the
__GNUC__ check so it works well when compiling with HP C using -Agcc
option that enables partial support for the GNU C dialect. The -Agcc
defines the __GNUC__ too.

Signed-off-by: Michal Rokos <michal.rokos@nextsoft.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>