Make e2fsprogs compilable with clang

clang's idea of "extern inline" is different from gcc's idea.
Use a variant that doesn't cause unresolved symbols for clang.

Change-Id: Ia8099139eaf818a6d0caec7c9d4ca579b3ed79d8
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>