libmraa: don't set SWIG_DIR

libmraa: don't set SWIG_DIR

CMake 3.0.12 uses SWIG_DIR to set SWIG_LIB, is used as the library
install location, which defaults to
$(STAGING_HOSTPKG)/share/sig/(SWIGVERSION).  If it is set, then the
installed swig library directory is ignored, and compilation fails:
[ 32%] Swig source
:1: Error: Unable to find 'swig.swg'
:3: Error: Unable to find 'python.swg'

Instead of setting it manually, let the default be used, which works
well.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>