add POSIX clocks and timers maintainer

Update the MAINTAINERS file: Thomas Gleixner has been the de-facto
maintainer of POSIX timers and clocks for quite some time.

Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

MAINTAINERS

···
 S:	Maintained
 
 POSIX CLOCKS and TIMERS
-P:	George Anzinger
-M:	[e-mail]
+P:	Thomas Gleixner
+M:	[e-mail]
 L:	[e-mail]
 S:	Supported