Fix cmake set_property in python bindings

`SET_SOURCE_FILES_PROPERTIES(pfw.i PROPERTIES SWIG_FLAGS "-v" "-Wall"
"-Werror")` sets 2 variables:
 - `SWIG_FLAGS="-v"`
 - `-Wall="-Werror"`
which is obviously not what was wanted.

Replace by `SET_PROPERTY(SOURCE pfw.i PROPERTY SWIG_FLAGS "-v" "-Wall"
"-Werror")` that only sets `SWIG_FLAGS="-v -Wall -Werror"`.

Signed-off-by: Kevin Rocard <kevin.rocard@intel.com>