Install gdk/win32 headers

Added install commands for the gdk/win32 headers as referenced from
gdkwin32.h.

https://bugzilla.gnome.org/show_bug.cgi?id=653964

gdk/win32/Makefile.am

@@ -2,6 +2,7 @@
 include $(top_srcdir)/Makefile.decl
 
 libgdkincludedir = $(includedir)/gtk-3.0/gdk
+libgdkwin32includedir = $(includedir)/gtk-3.0/gdk/win32
 
 AM_CPPFLAGS = \
 	-DG_LOG_DOMAIN=\"Gdk\"	\
@@ -65,4 +66,13 @@ libgdk_win32_la_SOURCES = \
 libgdkinclude_HEADERS =		\
 	gdkwin32.h
 
+libgdkwin32include_HEADERS =	\
+	gdkwin32cursor.h	\
+	gdkwin32display.h	\
+	gdkwin32displaymanager.h\
+	gdkwin32dnd.h		\
+	gdkwin32keys.h		\
+	gdkwin32screen.h	\
+	gdkwin32window.h
+
 -include $(top_srcdir)/git.mk