Don't add multiarch paths

The add_multiarch_paths() function leads, in certain build
environments, to the addition of host header paths to the CFLAGS,
which is not appropriate for cross-compilation. This patch fixes that
by simply removing the call to add_multiarch_paths() when we're
cross-compiling.

Investigation done by David <buildroot-2014@inbox.com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/setup.py
===================================================================
--- a/setup.py
+++ b/setup.py
@@ -454,9 +454,9 @@
         if not cross_compiling:
             add_dir_to_list(self.compiler.library_dirs, '/usr/local/lib')
             add_dir_to_list(self.compiler.include_dirs, '/usr/local/include')
+            self.add_multiarch_paths()
         if cross_compiling:
             self.add_gcc_paths()
-        self.add_multiarch_paths()
 
         # Add paths specified in the environment variables LDFLAGS and
         # CPPFLAGS for header and library files.