devtools: have symbol check script check for exported symbols

After last commit, our executables should export no symbols anymore.  To
make sure that this stays the case, verify this in the symbol checker
script.

contrib/devtools/symbol-check.py

@@ -98,10 +98,15 @@ if __name__ == '__main__':
     cppfilt = CPPFilt()
     retval = 0
     for filename in sys.argv[1:]:
+        # Check imported symbols
         for sym,version in read_symbols(filename, True):
             if version and not check_version(MAX_VERSIONS, version):
                 print('%s: symbol %s from unsupported version %s' % (filename, cppfilt(sym), version))
                 retval = 1
+        # Check exported symbols
+        for sym,version in read_symbols(filename, False):
+            print('%s: export of symbol %s not allowed' % (filename, cppfilt(sym)))
+            retval = 1
 
     exit(retval)