[FYI] Include <cmath> in dwarf-index-write.c

[Tom Tromey]

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>.
---
 gdb/ChangeLog           | 4 ++++
 gdb/dwarf-index-write.c | 1 +
 2 files changed, 5 insertions(+)

diff --git a/gdb/dwarf-index-write.c b/gdb/dwarf-index-write.c
index 3059e0b776..058712d9e2 100644
--- a/gdb/dwarf-index-write.c
+++ b/gdb/dwarf-index-write.c
@@ -34,6 +34,7 @@
 #include "psympriv.h"
 
 #include <algorithm>
+#include <cmath>
 #include <set>
 #include <unordered_map>
 #include <unordered_set>
-- 
2.13.6