libpng: use $() to reference make variables instead of ${}

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/libpng/libpng.mk

@@ -7,7 +7,7 @@
 LIBPNG_VERSION = 1.6.34
 LIBPNG_SERIES = 16
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.xz
-LIBPNG_SITE = http://downloads.sourceforge.net/project/libpng/libpng${LIBPNG_SERIES}/$(LIBPNG_VERSION)
+LIBPNG_SITE = http://downloads.sourceforge.net/project/libpng/libpng$(LIBPNG_SERIES)/$(LIBPNG_VERSION)
 LIBPNG_LICENSE = Libpng
 LIBPNG_LICENSE_FILES = LICENSE
 LIBPNG_INSTALL_STAGING = YES