package/matchbox-keyboard: add libpng dependency

libpng is a mandatory dependency since bump to version 0.1.1 in commit
e2511d88e83acae4144be89ce5e258b724067bd5 and
https://git.yoctoproject.org/matchbox-keyboard/commit/?id=1d7c5f1e0ad736205151c0df5aa738f942bd3e97:

checking for PNG... no
configure: error: Package requirements (libpng) were not met:

Package 'libpng', required by 'virtual:world', not found

Fixes:
 - http://autobuild.buildroot.org/results/4495f0720d7053a52ff8fef82d18a70a6f20c66b

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit a63fbb0d9762a0e6a454b1bea9ce44d620a6860a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/matchbox-keyboard/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_MATCHBOX_KEYBOARD
 	bool "matchbox-keyboard"
+	select BR2_PACKAGE_LIBPNG
 	select BR2_PACKAGE_XLIB_LIBXTST
 	select BR2_PACKAGE_XLIB_LIBXRENDER
 	select BR2_PACKAGE_XLIB_LIBXFT if !BR2_PACKAGE_CAIRO

package/matchbox-keyboard/matchbox-keyboard.mk

@@ -11,7 +11,7 @@ MATCHBOX_KEYBOARD_SITE = \
 MATCHBOX_KEYBOARD_LICENSE = LGPL-2.1, GPL-2.0+ (applet.c)
 MATCHBOX_KEYBOARD_LICENSE_FILES = COPYING applet/applet.c
 MATCHBOX_KEYBOARD_DEPENDENCIES = \
-	host-pkgconf matchbox-lib matchbox-fakekey expat xlib_libXrender
+	host-pkgconf matchbox-lib matchbox-fakekey expat libpng xlib_libXrender
 # From git
 MATCHBOX_KEYBOARD_AUTORECONF = YES