Add dependency on pixman

The driver now directly calls pixman, so add it as a pkgconfig
dependency.

Change-Id: I0249709b023afaaa093c3a3f27c159c2f4b7b64b
diff --git a/configure.ac b/configure.ac
index 4026ea6..bde2213 100644
--- a/configure.ac
+++ b/configure.ac
@@ -73,7 +73,14 @@
 XORG_DRIVER_CHECK_EXT(DPMSExtension, xextproto)
 
 # Checks for pkg-config packages
-PKG_CHECK_MODULES(XORG, [xorg-server >= 1.10] xproto fontsproto libdrm dri2proto $REQUIRED_MODULES)
+PKG_CHECK_MODULES(XORG,
+                  [xorg-server >= 1.10]
+                  xproto
+                  fontsproto
+                  libdrm
+                  dri2proto
+                  pixman-1
+                  $REQUIRED_MODULES)
 
 # Checks for header files.
 AC_HEADER_STDC