get.sh: properly initialise _ua

it is currently only initialised inside case
conditions. this is fine on most shells, but
some of them can be a bit buggy here.

initialise it empty and then override.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/get.sh

@@ -272,6 +272,8 @@ try_fetch_file()
 
 try_curl()
 {
+	_ua=""
+
 	case "$2" in
 	https://www.supermicro.com/*)
 		_ua="curl/8.6.0";;