Enable HAVE_DEV_TTY for Solaris

Now that git_terminal_prompt can cleanly interact with /dev/tty on
Solaris, enable HAVE_DEV_TTY so that this code path is used for
credential reading instead of relying on the crippled getpass().
Signed-off-by: NBen Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -909,6 +909,7 @@ ifeq ($(uname_S),SunOS)
 	NO_MKSTEMPS = YesPlease
 	NO_REGEX = YesPlease
 	NO_FNMATCH_CASEFOLD = YesPlease
+	HAVE_DEV_TTY = YesPlease
 	ifeq ($(uname_R),5.6)
 		SOCKLEN_T = int
 		NO_HSTRERROR = YesPlease