vbox: define DYNLIB_NAME for kFreeBSD

Similar to what Eric did for Cygwin it helps at least to compile
without --without-vbox

src/vbox/vbox_XPCOMCGlue.c

@@ -50,7 +50,9 @@
 /*******************************************************************************
 *   Defined Constants And Macros                                               *
 *******************************************************************************/
-#if defined(__linux__) || defined(__linux_gnu__) || defined(__sun__) || defined(__FreeBSD__) || defined(__OpenBSD__)
+#if defined(__linux__) || defined(__linux_gnu__) || defined(__sun__) || \
+    defined(__FreeBSD__) || defined(__OpenBSD__) || \
+    defined(__FreeBSD_kernel__)
 # define DYNLIB_NAME    "VBoxXPCOMC.so"
 #elif defined(__APPLE__)
 # define DYNLIB_NAME    "VBoxXPCOMC.dylib"