science/libecpint: Add explicit Python build dependency (D29353)

This unbreaks the build when using devel/samurai.

It previously got it through ninja which has a run dependency on
Python.

-- Could NOT find Python (missing: Python_EXECUTABLE Interpreter)
CMake Error at src/CMakeLists.txt:22 (file):
  file failed to open for reading (No such file or directory):

    /wrkdirs/usr/ports/science/libecpint/work/.build/src/qlist.txt

CMake Error at src/CMakeLists.txt:24 (add_custom_command):
  add_custom_command Wrong syntax.  A TARGET or OUTPUT must be specified.

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/libecpint-1.0.5.log

PR:		254678

science/libecpint/Makefile

@@ -12,7 +12,7 @@ LICENSE_FILE=	${WRKSRC}/LICENSE
 
 LIB_DEPENDS=	libpugixml.so:textproc/pugixml
 
-USES=		cmake compiler:c++11-lang
+USES=		cmake compiler:c++11-lang python:build
 USE_LDCONFIG=	yes
 
 USE_GITHUB=	yes