vnc-tls.h is included by vnc.h, and it includes gnutls/gnutls.h.
Hence, GnuTLS header files are needed by all files that include
vnc.h, most notably qmp.c.  Move these flags to QEMU_CFLAGS for
simplicity.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>