Add wireshark.pc.in to the source distribution (needed for cmake).

Bug: 10331
Change-Id: I81e0d224917cd4b38215e28ec146cd80efdb99d2
Reviewed-on: https://code.wireshark.org/review/3316
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
(cherry picked from commit 7f9fdbb8c10522d2bc31d285d1486601a441f09d)
Reviewed-on: https://code.wireshark.org/review/3317