Documentation/timers/hpet_example.c: only build on X86

We should only build hpet_example on x86[-64], where it is implemented.
It can cause build errors on other architectures.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/timers/Makefile

@@ -2,7 +2,7 @@
 obj- := dummy.o
 
 # List of programs to build
-hostprogs-y := hpet_example
+hostprogs-$(CONFIG_X86) := hpet_example
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)