[gdb/build] Fix build with gcc 7.5.0

When building gdb with gcc 7.5.0, I run into:
...
gdb/dwarf2/cooked-index.c: In lambda function:
gdb/dwarf2/cooked-index.c:104:5: error: \
  the value of ‘_sch_tolower’ is not usable in a constant expression
     };
     ^
In file included from gdbsupport/gdb-safe-ctype.h:47:0,
                 from gdb/dwarf2/cooked-index.c:34:
include/safe-ctype.h:111:29: note: ‘_sch_tolower’ was not declared ‘constexpr’
 extern const unsigned char  _sch_tolower[256];
                             ^~~~~~~~~~~~
...

This does not happen with gcc 8.2.1.

Fix this by dropping the constexpr on lambda function munge in
cooked_index_entry::compare for gcc 7.

Tested by completing the build on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>