[binutils-gdb] Add ATTRIBUTE_PRINTF to trace_start_error

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 1d4fbac99e05c2d2ea98984f9bc6f7d7f1a183b7 ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: master
Commit: 1d4fbac99e05c2d2ea98984f9bc6f7d7f1a183b7

Add ATTRIBUTE_PRINTF to trace_start_error

clang complains that the fmt passed to vwarning in trace_start_error is
not a literal.  This looks like a fair warning, which can be removed by
adding ATTRIBUTE_PRINTF to the declaration of trace_start_error.

gdb/ChangeLog:

	* nat/fork-inferior.h (trace_start_error): Add ATTRIBUTE_PRINTF.