MythTV: Fix how configure finds libxml2.

Remove the first component of the include file name as it is redundant
with the path provided by pkg-config.  This is not normally noticable
as on most systems the default include path contains the parent of the
libxml include path, and it all works out.  On FreeBSD this is not the
case, and the libxml2 libraries can't be found without tweaking.

Signed-off-by: Peter Bennett <pbennett@mythtv.org>

mythtv/external/FFmpeg/configure

@@ -6486,7 +6486,7 @@ enabled libzmq            && require_pkg_config libzmq "libzmq >= 4.2.1" zmq.h z
 enabled libzvbi           && require_pkg_config libzvbi zvbi-0.2 libzvbi.h vbi_decoder_new &&
                              { test_cpp_condition libzvbi.h "VBI_VERSION_MAJOR > 0 || VBI_VERSION_MINOR > 2 || VBI_VERSION_MINOR == 2 && VBI_VERSION_MICRO >= 28" ||
                                enabled gpl || die "ERROR: libzvbi requires version 0.2.28 or --enable-gpl."; }
-enabled libxml2           && require_pkg_config libxml2 libxml-2.0 libxml2/libxml/xmlversion.h xmlCheckVersion
+enabled libxml2           && require_pkg_config libxml2 libxml-2.0 libxml/xmlversion.h xmlCheckVersion
 enabled mbedtls           && { check_pkg_config mbedtls mbedtls mbedtls/x509_crt.h mbedtls_x509_crt_init ||
                                check_pkg_config mbedtls mbedtls mbedtls/ssl.h mbedtls_ssl_init ||
                                check_lib mbedtls mbedtls/ssl.h mbedtls_ssl_init -lmbedtls -lmbedx509 -lmbedcrypto ||