Add ninja to Cygwin builder

Since there are no contourpy wheels, it must be built from source, and
pre-packaged ninja is better than trying to build from sdist.

Author: QuLogic

.github/workflows/cygwin.yml

@@ -86,7 +86,7 @@ jobs:
         with:
           packages: >-
             ccache gcc-g++ gdb git graphviz libcairo-devel libffi-devel
-            libgeos-devel libQt5Core-devel pkgconf libglib2.0-devel
+            libgeos-devel libQt5Core-devel pkgconf libglib2.0-devel ninja
             noto-cjk-fonts
             python3${{ matrix.python-minor-version }}-devel
             python3${{ matrix.python-minor-version }}-pip