gnutls: Do not use libunistring prefix

when using clang, configure it poking at build host
if we do not use it then it falls back to sysroot
which is what we need here.

(From OE-Core rev: ee0b0da237c1661f43e27e26e0ab24b7d8ed174f)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-support/gnutls/gnutls.inc

@@ -37,6 +37,7 @@ EXTRA_OECONF = " \
     --enable-local-libopts \
     --enable-openssl-compatibility \
     --with-libpthread-prefix=${STAGING_DIR_HOST}${prefix} \
+    --without-libunistring-prefix \
 "
 
 LDFLAGS_append_libc-musl = " -largp"