src: Remove $(builddir) usage

Commit 4e8032272f used $(builddir) in the header search
path to fix a build issue; however, $(builddir) is not defined
by old autoconf versions such as the one available in CentOS 5,
resulting in the following error:

  cc1: error: /util: No such file or directory
  make[3]: *** [libvirt_driver_la-fdstream.lo] Error 1

Since $(builddir) is defined to always be '.', just use that
value directly instead.

src/Makefile.am

@@ -31,7 +31,7 @@ INCLUDES =	-I../gnulib/lib					\
 		-I../include					\
 		-I$(top_srcdir)/include				\
 		-I$(srcdir)/util				\
-		-I$(builddir)/util				\
+		-I./util					\
 		-DIN_LIBVIRT					\
 		-Dabs_topbuilddir="\"$(abs_topbuilddir)\""	\
 		-Dabs_topsrcdir="\"$(abs_topsrcdir)\""		\