Fix help message of apinames.

* src/tools/apinames.c: The first header file must be
  given after the options.  All arguments after the
  first header file are dealt as header files to be
  parsed, regardless with their leading "-".

  For example,
  "apinames include/freetype/freetype.h -dFREETYPE.DLL"
  warns "unable to open -dFREETYPE.DLL".

  Thus, the "header1" must be given after the arguments
  to be parsed as the options.

src/tools/apinames.c

@@ -380,7 +380,7 @@ usage( void )
     "It receives a list of header files as an argument and\n"
     "generates a sorted list of unique identifiers in various formats.\n"
     "\n"
-    "usage: %s header1 [options] [header2 ...]\n"
+    "usage: %s [options] header1 [header2 ...]\n"
     "\n"
     "options:   -       parse the contents of stdin, ignore arguments\n"
     "           -v      verbose mode, output sent to standard error\n"