ext: Add pybind rev f4b81b3

Change-Id: I52e4fc9ebf2f59da57d8cf8f3e37cc79598c2f5f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Hansson <andreas.hansson@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2229
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Pierre-Yves Péneau <pierre-yves.peneau@lirmm.fr>

SConstruct

@@ -1381,6 +1381,8 @@ for root, dirs, files in os.walk(ext_dir):
         main.SConscript(joinpath(root, 'SConscript'),
                         variant_dir=joinpath(build_root, build_dir))
 
+main.Prepend(CPPPATH=Dir('ext/pybind11/include/'))
+
 ###################################################
 #
 # This function is used to set up a directory with switching headers