Fix issue where include file checking was skipped
Fix issue where include file checking was skipped

Fixes small bug in the custom extension changes. The dot was ommitted
from header extensions so the header file was never found. In addition,
a function that should have checked only source files (like .cc)
actually checked all files, so header files were being checked to see if
they included their headers.

theandrewdavis/cpplint