net: pch_gbe depends on x86

Since 4bb16672

 (build some drivers only
when compile-testing), PTP_1588_CLOCK_PCH depends on (X86 ||
COMPILE_TEST). But PCH_GBE selects PTP_1588_CLOCK_PCH without
depending on x86. Fix this by adding the same dependency here.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: <fengguang.wu@intel.com> [intel's build test robot]
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: netdev@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: kbuild-all@01.org
Signed-off-by: David S. Miller <davem@davemloft.net>