Include <cmath> in dwarf-index-write.c

Include <cmath> in dwarf-index-write.c

On x86-64 Fedora 26, when building with the system gcc, I get:

../../binutils-gdb/gdb/dwarf-index-write.c: In member function ‘void debug_names::build()’:
../../binutils-gdb/gdb/dwarf-index-write.c:705:13: error: ‘pow’ is not a member of ‘std’

There are actually more messages, but this is sufficient to show the
problem.

The fix is to include <cmath>.

I'm checking this in as obvious.  Tested by building.

gdb/ChangeLog
2018-03-27  Tom Tromey  <tom@tromey.com>

	* dwarf-index-write.c: Include <cmath>.