rtl-sdr: remove -I/usr/include from pkg-config

rtl-sdr: remove -I/usr/include from pkg-config

The -I/usr/include in the .pc file for rtl-sdr causes build failures
with GCC 6 in qtmultimedia-rtlfm-radio-plugin because QMake translates
it into -isystem, which breaks the GCC 6 #include_next directive. More
information is available here:

Here's some references in case you're curious:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
https://bugreports.qt.io/browse/QTBUG-53367
https://bugreports.qt.io/browse/QTBUG-53375
https://codereview.qt-project.org/#/c/159394

Change-Id: I2fa58c422323026bcf254491b9ee7e30e8e75938
Signed-off-by: Martin Kelly <mkelly@xevo.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9167
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>