[CMake] put python header in include

Otherwise, this header is installed if BUILD_PYTHON_INTERFACE, and not
if INSTALL_PYTHON_INTERFACE_ONLY.

But if we want to be able to have binary packages for:
- the main curve package
- its bindings for python 2
- its bindings for python 3

this header will be installed in the 2 last cases, which will conflict.

In short: if we set INSTALL_PYTHON_INTERFACE_ONLY, there should be only
stuff installed in PYTHON_SITELIB.
9 jobs for v0.3.2 in 4 minutes and 28 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
canceled doc-coverage #93838

00:04:27

canceled format #93839
allowed to fail

00:04:27

canceled robotpkg-curves-18.04-release #93831

00:04:28

failed robotpkg-py-curves-py2-18.04-release #93834

00:01:19

canceled robotpkg-py-curves-py3-16.04-release #93832
allowed to fail

00:04:28

canceled robotpkg-py-curves-py3-18.04-release #93833

00:04:28

canceled robotpkg-py-curves-py3-20.04-release #93835

00:04:28

failed robotpkg-py-curves-py3-buster-release #93836

00:01:25

canceled robotpkg-py-curves-py3-stretch-release #93837
allowed to fail

00:04:27

 
Name Stage Failure
failed
robotpkg-py-curves-py3-buster-release Test
ERROR: searching in /opt/openrobots
ERROR: found: /opt/openrobots/include/curves/config.hh, wrong version 0.3.2
ERROR: missing: /opt/openrobots/include/curves/config.hh
ERROR:
ERROR: Please reinstall the package in ../../math/curves
ERROR: ======================================================================
make: *** [/root/robotpkg/mk/depends/depends.mk:70: do-depends] Error 1
make: *** [/root/robotpkg/mk/internal/trusted.mk:36: install] Error 2
ERROR: Job failed: exit code 1
failed
robotpkg-py-curves-py2-18.04-release Test
ERROR: missing:	/opt/openrobots/include/curves/config.hh
ERROR:
ERROR: Please reinstall the package in ../../math/curves
ERROR: ======================================================================
/root/robotpkg/mk/depends/depends.mk:70: recipe for target 'do-depends' failed
make: *** [do-depends] Error 1
/root/robotpkg/mk/internal/trusted.mk:36: recipe for target 'install' failed
make: *** [install] Error 2
ERROR: Job failed: exit code 1