build: define 'inline' iff HAVE_LIBNL1

Previous commit 0b4b53bb defined 'inline' to prevent broken build on
systems with libnl1 headers. However, it broke build on systems with
libnl3 headers. Therefore we must make that fix conditional.

src/util/virnetlink.h

@@ -28,9 +28,13 @@
 
 /* Work around a bug where older libnl-1 headers expected older gcc
  * semantics of 'extern inline' that conflict with C99 semantics.  */
-#  define inline
+#  ifdef HAVE_LIBNL1
+#   define inline
+#  endif
 #  include <netlink/msg.h>
-#  undef inline
+#  ifdef HAVE_LIBNL1
+#   undef inline
+#  endif
 
 # else