[gn build] attempt to unbreak linux after fc9696130c8

Only emit `global:` if there are any exported symbols.

While here, `chmod +x` the symbol_exports.py script.

utils/gn/build/symbol_exports.py

@@ -25,11 +25,12 @@ def main():
     symbols = open(args.source).readlines()
 
     if args.format == 'linux':
-      output_lines = (['FOO {\n',
-                       '  global:\n',] +
-                      ['    %s;\n' % s.rstrip() for s in symbols] +
-                      ['  local: *;\n',
-                       '}\n'])
+      output_lines = ['FOO {\n']
+      if symbols:
+        output_lines += (['  global:\n',] +
+                         ['    %s;\n' % s.rstrip() for s in symbols])
+      output_lines += ['  local: *;\n',
+                       '}\n']
     elif args.format == 'mac':
       output_lines = ['_' + s for s in symbols]
     else: