build/win: Mark poppler as optional

Compiling it tends to fail on newer mingw releases.

build/windows/jhbuild/gimp.moduleset

@@ -42,12 +42,12 @@
 		<dep package="iso-codes"/>
 		<dep package="libwmf"/>
 		<dep package="ghostscript"/>
-		<dep package="poppler"/>
 	</dependencies>
 	<suggests>
 		<dep package="webkitgtk"/>
 		<dep package="libmng"/>
 		<dep package="gdb"/>
+		<dep package="poppler"/>
 	</suggests>
 </metamodule>