Distribute libvirt_vmx.syms

This fixes the build from a tarball and makes autobuild.sh
work again.

This should actually have been part of this earlier commit:

  esx: Move VMX handling code out of the driver directory
  42b2f35d

Reported by Eric Blake.

src/Makefile.am

@@ -1066,7 +1066,8 @@ EXTRA_DIST += \
   libvirt_linux.syms		\
   libvirt_macvtap.syms		\
   libvirt_daemon.syms		\
-  libvirt_nwfilter.syms
+  libvirt_nwfilter.syms	\
+  libvirt_vmx.syms
 
 BUILT_SOURCES += libvirt.syms libvirt.def libvirt_qemu.def