check-po.awk: fix awk invocation

The awk utility has been moved to /usr/bin and the script relies on
features present in gawk but not in mawk.

tools/README

@@ -68,7 +68,9 @@ Maybe useful for SANE developers:
  check-po.awk:
         Print untranslated and fuzzy messages and their line numbers in the
         source code and po file. Example:
-           cd po/ ; ../tools/check-po.awk sane-backends.de.po
+           tools/check-po.awk po/de.po
-        You may need to adjust the path of awk in the first line of the
+        You may need to adjust the path of gawk in the first line of the
-        script. More documentation is in the script itself.
+        script. Alternatively, run it via gawk's -f option, like so:
+           gawk -f tools/check-po.awk po/de.po
+        More documentation is in the script itself.
 

tools/check-po.awk

@@ -1,4 +1,4 @@
-#!/bin/awk -f 
+#!/usr/bin/gawk -f
 
 #  This program is free software; you can redistribute it and/or
 #  modify it under the terms of the GNU General Public License as