MAINTAINERS: Change my email address to the new domain

My old email address will soon not work anymore, so change it to the
new domain.
Signed-off-by: NStefan Berger <stefanb@linux.ibm.com>

MAINTAINERS

@@ -1794,7 +1794,7 @@ F: docs/devel/tracing.txt
 T: git git://github.com/stefanha/qemu.git tracing
 
 TPM
-M: Stefan Berger <stefanb@linux.vnet.ibm.com>
+M: Stefan Berger <stefanb@linux.ibm.com>
 S: Maintained
 F: tpm.c
 F: stubs/tpm.c