configure: fix linking with statically linked freetype

freetype depends on zlib. using statically linked freetype as system
library lacks the usage requirement to link with zlib. so we need to
add this manually.

Task-number: QTBUG-63115
Change-Id: Iaf0f3027bd9d1386fcc1ecfbfbe07ab09b2d0bb9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/gui/configure.json

@@ -164,6 +164,9 @@
                 { "type": "pkgConfig", "args": "freetype2" },
                 { "type": "freetype", "libs": "-lfreetype", "condition": "!config.wasm" },
                 { "type": "freetype", "libs": "-s USE_FREETYPE=1", "condition": "config.wasm" }
+            ],
+            "use": [
+                { "lib": "zlib", "condition": "features.system-zlib" }
             ]
         },
         "fontconfig": {