dtoolbase: Rename pdtoa.c -> pdtoa.cxx.

It's actually a C++ file (contains C++-only constructs) and should have the proper file extension so it compiles with the appropriate language.

dtool/src/dtoolbase/p3dtoolbase_composite2.cxx

@@ -2,7 +2,7 @@
 #include "mutexWin32Impl.cxx"
 #include "mutexSpinlockImpl.cxx"
 #include "neverFreeMemory.cxx"
-#include "pdtoa.c"
+#include "pdtoa.cxx"
 #include "pstrtod.cxx"
 #include "register_type.cxx"
 #include "typeHandle.cxx"