MAINTAINERS: update Vineet's email address

I'll be leaving Synopsys shortly, but will continue to handle maintenance
for the transition period.
Signed-off-by: NVineet Gupta <vgupta@kernel.org>

MAINTAINERS

@@ -17815,7 +17815,7 @@ F:	include/linux/sync_file.h
 F:	include/uapi/linux/sync_file.h
 
 SYNOPSYS ARC ARCHITECTURE
-M:	Vineet Gupta <vgupta@synopsys.com>
+M:	Vineet Gupta <vgupta@kernel.org>
 L:	linux-snps-arc@lists.infradead.org
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vgupta/arc.git