wscript: ensure nanopb is configured when building with --onlysdk

Fixes builds failing with --onlysdk due to missing nanopb headers in the include
path.

Signed-off-by: Ruby Iris Juric <ruby@srxl.me>

wscript

@@ -713,6 +713,8 @@ def build(bld):
         return
 
     if bld.options.onlysdk:
+        # required to build src/idl
+        bld.recurse('third_party/nanopb')
         # stop here, sdk generation is done
         return