Remove warning about tail "potentially uninitialized"

MSVC was complaining that the `tail` variable was potentially
uninitialized in the while branch. Since the while loop is actually
coupled to the if (head != NULL) that initializes the tail variable,
we move them together, which makes the warning disappear.

tools/windows/hammer_lib_src_list

@@ -3,7 +3,8 @@ allocator.c
 benchmark.c
 bitreader.c 
 bitwriter.c 
-cfgrammar.c 
+cfgrammar.c
+datastructures.c
 desugar.c 
 glue.c 
 hammer.c