Since commit 70dc8a48 (checkpatch: warn when using extern with function
prototypes in .h files), we will get checkpatch warning when updating
common.h following the existing convention which has extern for function
prototypes.

Let's change the convention to not use extern with function prototypes
in this header.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>