package/gstreamer1/gst1-python: fix libpython-dir path

package/gstreamer1/gst1-python: fix libpython-dir path

Due to the CONF_ENV options set in the gst1-python.mk file, libpython-dir
must be set manually or else the error:
"Python dynamic library path could not be determined" occurs.

Previously the libpython-dir option was set to
$(HOST_DIR)/lib/python$(PYTHON3_VERSION_MAJOR), however, this breaks the
resulting .so because the above full path is baked into the resulting .so.
which results in the error:

Couldn't g_module_open libpython. Reason:
/full/path/to/host/lib/python3.8/libpython3.8.so: cannot open shared object
file: No such file or directory

To fix this error, set the libpython-dir to /usr/lib.
Because we provide PYTHONPATH=$(PYTHON3_PATH) in the GST1_PYTHON_CONF_ENV,
the logic in the meson file uses the above python3 provided by the PYTHONPATH
variable to determine /usr/lib/ has the proper
python$(PYTHON3_VERSION_MAJOR).so file.

Because Buildroot provides the appropriate paths, the meson file finds
the correct .so file and the resulting compiled library has the appropriate
path of /usr/lib/python3.$(PYTHON3_VERSION_MAJOR).so

This change has been tested on the following distributions:
  - Debian 9 and 10
  - Debian 9 without python3 installed on the host.
  - Centos7
  - Fedora 31

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>