cups: fix static linking with GnuTLS

cups: fix static linking with GnuTLS

cups forgets to link against the libs found by pkg-config if it is build with
GnuTLS support.

Otherwise, static build fails with:
../cups/libcups.a(tls.o): In function `http_gnutls_create_credential':
cups-2.1.2/cups/tls-gnutls.c:762: undefined reference to `gnutls_x509_crt_init'
[..]

Fixes:
http://autobuild.buildroot.net/results/205/20509b7d1edc73dc2a7622f2ae7dbf9f6665a918/
http://autobuild.buildroot.net/results/b86/b86d685caf8cf812f9505fca2de36fb8c18baee1/

Patch is sent upstream.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
[Thomas: refresh and rename patch.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>