Darwin, D: Fix D bootstrap, include tm-dwarf2.h.

After r12-4432-g7bfe7d634f60b0a9 Darwin fails to bootstrap with D
enabled since there is no definition of either DWARF2_DEBUG_INFO or
PREFERRED_DEBUGGING_TYPE.

Fixed here by adding the tm-dwarf2.h file to tm_d_file for Darwin.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/ChangeLog:

	* config.gcc: Add tm-dwarf2.h to tm_d-file.
diff --git a/gcc/config.gcc b/gcc/config.gcc
index aa5bd5d..3675e06 100644
--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -666,6 +666,7 @@
 *-*-darwin*)
   tmake_file="t-darwin "
   tm_file="${tm_file} darwin.h"
+  tm_d_file="${tm_d_file} tm-dwarf2.h"
   darwin_os=`echo ${target} | sed 's/.*darwin\([0-9.]*\).*$/\1/'`
   darwin_maj=`expr "$darwin_os" : '\([0-9]*\).*'`
   macos_min=`expr "$darwin_os" : '[0-9]*\.\([0-9]*\).*'`