Fix libpng linkage - shared on all platforms other than iOS

conanfile.py

@@ -47,7 +47,7 @@ class VCMI(ConanFile):
         self.options["freetype"].shared = self.settings.os == "Android"
 
         # SDL_image and Qt depend on it, in iOS both are static
-        self.options["libpng"].shared = not self.settings.os != "iOS"
+        self.options["libpng"].shared = self.settings.os != "iOS"
         # static Qt for iOS is the only viable option at the moment
         self.options["qt"].shared = self.settings.os != "iOS"