Cast the log10 argument to double to disambiguate it

Cast the log10 argument to double to disambiguate it

On Solaris 11 with gcc 5.5.0 (gcc211 on the compile farm), math.h has a
using std::log10; directive. This is unfortunate because std::log10 has
overloads for float/double/long double. To disambiguate this call,
cast the argument to double to fix the build.

gdb/ChangeLog:

2019-12-19  Christian Biesinger  <cbiesinger@google.com>

	* tui/tui-source.c (tui_source_window::set_contents): Cast argument of
	log10 to double to fix Solaris 11 with gcc 5.5.

Change-Id: I6c0c52e9c172b529c899a435d430e5916aeef69f