add explicit png build dependency to xpdfimport

src/sfnt/pngshim.c:298: error: undefined reference to 'png_create_read_struct'

Change-Id: Iafb7b685f09347f711b9221362f999b3d48c9bda
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/167656
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>

sdext/Executable_xpdfimport.mk

@@ -13,6 +13,7 @@ $(eval $(call gb_Executable_use_externals,xpdfimport,\
     boost_headers \
     expat \
     freetype \
+    libpng \
     poppler \
     $(if $(filter-out WNT MACOSX,$(OS)),fontconfig) \
     zlib \