Add cmake/ to MANIFEST.in (openPMD#1140)

Forgot to add in openPMD#1128, which is then missing in Python sdist files
for pip from-source builds.

MANIFEST.in

@@ -2,6 +2,7 @@ include README.md COPYING COPYING.LESSER
 include pyproject.toml
 include requirements.txt
 global-include CMakeLists.txt *.cmake *.in
+recursive-include cmake *
 recursive-include include *
 recursive-include src *
 recursive-include share *