[dev-libs/libbson] fix USE=verify-sig

dev-libs/libbson/libbson-1.30.6.ebuild

@@ -50,6 +50,13 @@ PATCHES=(
 	"${FILESDIR}"/libbson-1.30.6-cmake4.patch
 )
 
+src_unpack() {
+        if use verify-sig; then
+                verify-sig_verify_detached "${DISTDIR}"/${P}.tar.gz{,.asc}
+        fi
+        default
+}
+
 src_prepare() {
 	cmake_src_prepare