[binutils-gdb] linux-waitpid: Get rid of format string warning

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 1e5b66ed4285ecf063e7801a46178ebb0ce5be41 ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: master
Commit: 1e5b66ed4285ecf063e7801a46178ebb0ce5be41

linux-waitpid: Get rid of format string warning

Clang gives this warning:

/home/emaisin/src/binutils-gdb/gdb/gdbserver/../nat/linux-waitpid.c:45:25: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
      vfprintf (stderr, format, args);
                        ^~~~~~

Get rid of it by adding ATTRIBUTE_PRINTF.

gdb/ChangeLog:

	* nat/linux-waitpid.c (linux_debug): Add ATTRIBUTE_PRINTF.