flatpak: Use the modern buildsystem property

https://bugzilla.gnome.org/show_bug.cgi?id=788469

org.gnome.Calendar.json

@@ -45,7 +45,7 @@
         {
             "name": "libical",
             "cleanup": [ "/lib/cmake"],
-            "cmake": true,
+            "buildsystem": "cmake",
             "config-opts": [ "-DCMAKE_INSTALL_LIBDIR:PATH=/app/lib",
                              "-DBUILD_SHARED_LIBS:BOOL=ON" ],
             "sources": [
@@ -64,6 +64,7 @@
                 "/libexec",
                 "/share/dbus-1/services"
 	    ],
+            "buildsystem": "cmake",
             "config-opts": [
                 "-DENABLE_GTK=ON",
                 "-DENABLE_GOOGLE_AUTH=OFF",
@@ -77,7 +78,6 @@
                 "-DENABLE_GTK_DOC=OFF",
                 "-DENABLE_EXAMPLES=OFF"
             ],
-            "cmake": true,
             "sources": [
                 {
                     "type": "git",