package/selinux-python: fix dependency

package/selinux-python: fix dependency

Commit 791292c48d5165515ca08f35d8cbc1ef78d3ed95 forgot to add
!BR2_PACKAGE_PYTHON dependency, without it the following error will be
raised if the user selects BR2_PACKAGE_PYTHON and python-selinux:

WARNING: unmet direct dependencies detected for BR2_PACKAGE_PYTHON3
  Depends on [n]: !BR2_PACKAGE_PYTHON [=y] && BR2_USE_WCHAR [=y] && BR2_USE_MMU [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && !BR2_STATIC_LIBS [=n]
  Selected by [y]:
  - BR2_PACKAGE_SELINUX_PYTHON [=y] && BR2_USE_MMU [=y] && BR2_USE_WCHAR [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && !BR2_STATIC_LIBS [=n]

Fixes:
 - No autobuilder failures yet

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: add dependency on MMU]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>