configure: don't enable ENABLE_NPAPI_FROM_BROWSER with --enable-headless

This fix a build error since otherwise X libraries are required
Backport of 759cc275

Change-Id: Iba718969dca90563ba9e60aa16944e6de67c7141
Reviewed-on: https://gerrit.libreoffice.org/3786Reviewed-by: Andras Timar <atimar@suse.com>
Tested-by: Andras Timar <atimar@suse.com>

configure.ac

@@ -8418,7 +8418,7 @@ dnl ===================================================================
 dnl Check for NPAPI interface to plug browser plugins into LibreOffice documents
 dnl ===================================================================
 AC_MSG_CHECKING([whether to plug browser plugins into LibreOffice documents])
-if test "$_os" != Android -a "$_os" != iOS
+if test "$_os" != Android -a "$_os" != iOS -a "$enable_headless" != yes
 then
     AC_MSG_RESULT([yes])
     ENABLE_NPAPI_FROM_BROWSER=YES