gdb: fix some #ifdef logic in bt-utils.h

gdb: fix some #ifdef logic in bt-utils.h

In passing I spotted some incorrect #ifdef logic in bt-utils.h.  The
logic in question has existed since the file was originally added in
commit:

  commit abbbd4a3e0ca51132e7fb31a43f896d29894dae0
  Date:   Wed Aug 11 13:24:33 2021 +0100

      gdb: use libbacktrace to create a better backtrace for fatal signals

The code is trying to select between using libbacktrace or using the
execinfo supplied backtrace API.

First we check to see if we can use libbacktrace.  If we can then we
include some header files, and then set some defines to indicate that
libbacktrace is being used.

Then we check if execinfo is available, if it is then we include
<execinfo.h> and set some alternative defines.

In theory the second block of logic should not trigger if the first
block (that uses libbacktrace) has also triggered, but we incorrectly
check the define 'PRINT_BACKTRACE_ON_FATAL_SIGNAL' instead of checking
for 'GDB_PRINT_INTERNAL_BACKTRACE_USING_LIBBACKTRACE', so the second
block triggers more than it should.  The
'PRINT_BACKTRACE_ON_FATAL_SIGNAL' define is not defined anywhere, this
was a mistake in the original commit.

In reality this is harmless, we include <execinfo.h> when we don't
need too, but in by-utils.c the libbacktrace define is always checked
for before the execinfo define, so we never actually end up using the
execinfo path (when libbacktrace is available).  But I figure its
still worth cleaning this up.

I've tested GDB in a "default" build where libbacktrace is used, and
when configuring with --disable-libbacktrace which causes the execinfo
backtrace API to be used instead, both still appear to work fine.

There should be no user visible changes after this commit.