FindFFmpeg: Use the right name for the pkg-config component variables.

Commit 3194a35e ("Rewritten FindFFmpeg.cmake") introduced a regression
in the find_component macro: the pkg_check_module() call uses
PC_${component} as the variable name, while the calls to find_path() and
find_library() use PC_LIB${component} instead.

Fix the calls to find_path() and find_library().

Based on a patch by Thomas Klausner <tk@giga.or.at>, thanks a lot!

Copied from kdelibs commit f6d33b3a2d1077c91425819de145ba2e5ae3cd87

BUG: 338280
FIXED-IN: 4.14.1