stv0991: record defconfig ownership in MAINTAINERS

This commit fixes warnings reported by tools/genboardscfg.py.
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Vikas Manocha <vikas.manocha@st.com>
Acked-by: NVikas Manocha <vikas.manocha@st.com>

board/st/stv0991/MAINTAINERS

@@ -3,3 +3,4 @@ M:	Vikas Manocha <vikas.manocha@st.com>
 S:	Maintained
 F:	board/st/stv0991/
 F:	include/configs/stv0991.h
+F:	configs/stv0991_defconfig