devel/tinygo: 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 PythonInterp (missing: PYTHON_EXECUTABLE)
CMake Error at CMakeLists.txt:652 (message):
  Unable to find Python interpreter, required for builds and testing.

  Please install Python or specify the PYTHON_EXECUTABLE CMake variable.

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/tinygo-0.14.1_1.log

PR:		254678

devel/tinygo/Makefile

@@ -15,7 +15,7 @@ BUILD_DEPENDS=	cmake:devel/cmake \
 		ninja:devel/ninja
 TEST_DEPENDS=	qemu-system-arm:emulators/qemu
 
-USES=		gmake go:modules,run
+USES=		gmake go:modules,run python:build
 
 USE_GITHUB=	yes
 GH_ACCOUNT=	tinygo-org