Added missing dependency for Complie error on RHEL7

Complie error on RHEL7 for missing depency libos.

unix/xserver/hw/vnc/Makefile.am

@@ -3,9 +3,10 @@ LIB_DIR=${top_builddir}/../../common
 
 RFB_LIB=$(LIB_DIR)/rfb/librfb.la
 RDR_LIB=$(LIB_DIR)/rdr/librdr.la
+OS_LIB=$(LIB_DIR)/os/libos.la
 NETWORK_LIB=$(LIB_DIR)/network/libnetwork.la
 XREGION_LIB=$(LIB_DIR)/Xregion/libXregion.la
-COMMON_LIBS=$(NETWORK_LIB) $(RFB_LIB) $(RDR_LIB) $(XREGION_LIB)
+COMMON_LIBS=$(NETWORK_LIB) $(RFB_LIB) $(RDR_LIB) $(XREGION_LIB) $(OS_LIB)
 
 noinst_LTLIBRARIES = libvnccommon.la