Remove empty variable SHARK_COMMON_CAPTURE_SRC. It's not present
for the other build systems as well.
Small whitespace fix.

Change-Id: I071ee4ae0fa85306435d2d82d36265973181b71c
Reviewed-on: https://code.wireshark.org/review/3263
Reviewed-by: Jörg Mayer <jmayer@loplof.de>