package/boinc: add optional dependency to freetype

No configure option present:
https://github.com/BOINC/boinc/blob/client_release/7.8/7.8.3/configure.ac#L497

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/boinc/boinc.mk

@@ -26,6 +26,10 @@ BOINC_CONF_OPTS = \
 	--with-pkg-config=$(PKG_CONFIG_HOST_BINARY) \
 	--with-libcurl=$(STAGING_DIR)/usr
 
+ifeq ($(BR2_PACKAGE_FREETYPE),y)
+BOINC_DEPENDENCIES += freetype
+endif
+
 ifeq ($(BR2_PACKAGE_LIBFCGI),y)
 BOINC_DEPENDENCIES += libfcgi
 BOINC_CONF_OPTS += --enable-fcgi