build: help Haiku find the header files that define mkstemps()

The documentation (https://www.haiku-os.org/docs/api/libroot.html)
says that BSD extensions are enabled by default, but somehow this
does not seem to be the case on my system.  With _DEFAULT_SOURCE
defined, though, src/files.c now compiles fine.

src/definitions.h

@@ -29,6 +29,10 @@
 #endif
 #endif
 
+#if defined(__HAIKU__) && !defined(_DEFAULT_SOURCE)
+#define _DEFAULT_SOURCE  1
+#endif
+
 #ifdef __TANDEM
 /* Tandem NonStop Kernel support. */
 #include <floss.h>