pal_linux.h is missing include guard #endif

The recent change to pal_linux.h removed the #endif, breaking the linux-acs build.

/home/rebecca/arm-enterprise-acs/luv/build/tmp/work/qemuarm64-oe-linux/sbsa-acs-pal/1.0-r0/include/pal_linux.h:22:0: error: unterminated #ifndef
  #ifndef __PAL_LINUX_H__

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