Exclude 2 gtest files from translation (#781978)

Running 'make check' is failing, reporting that 2 of the Google Test
files need to be translated.

    $ make check
    ...
    Making check in po
    make[1]: Entering directory `/home/centos/programming/c/gparted/po'
    rm -f missing notexist
    srcdir=. /usr/bin/intltool-update -m
    The following files contain translations and are currently not in use. Please
    consider adding these to the POTFILES.in file, located in the po/ directory.

    lib/gtest/include/gtest/internal/gtest-filepath.h
    lib/gtest/src/gtest.cc

    If some of these files are left out on purpose then please add them to
    POTFILES.skip instead of POTFILES.in. A file 'missing' containing this list
    of left out files has been written in the current directory.
    Please report to https://bugzilla.gnome.org/enter_bug.cgi?product=gparted
    if [ -r missing -o -r notexist ]; then \
      exit 1; \
    fi
    make[1]: *** [check] Error 1
    make[1]: Leaving directory `/home/centos/programming/c/gparted/po'
    make: *** [check-recursive] Error 1
    'make check' is failing

This is not true, the files do not contain translatable strings.  This
is a match for known intltool bug:

    False positives from intltool-update -m
    https://bugs.launchpad.net/intltool/+bug/545862

Fix by adding the problem file names into POTFILES.skip.

Bug 781978 - Add Google Test C++ test framework

po/POTFILES.skip

@@ -1,5 +1,7 @@
 # List of source files containing translatable strings, but should be skipped.
 # Please keep this file sorted alphabetically.
 gparted.desktop.in
+lib/gtest/include/gtest/internal/gtest-filepath.h
+lib/gtest/src/gtest.cc
 # https://bugs.launchpad.net/intltool/+bug/1117944
 sub/gparted.desktop.in