expect: fix native build with GCC 15

This does not work for native builds:
  CFLAGS += "-std=gnu17"
The line from native.bbclass gets priority:
  CFLAGS = "${BUILD_CFLAGS}"

From bitbake-getvar -r expect-native CFLAGS
  ...
  append ...poky/meta/recipes-devtools/expect/expect_5.45.4.bb:44
     "-std=gnu17"
  set ...poky/meta/classes-recipe/native.bbclass:44
     "${BUILD_CFLAGS}"
  ...

(From OE-Core rev: 2696c50af9946f425ccaf7d0e7e0eb3fd87c36bb)

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-devtools/expect/expect_5.45.4.bb

@@ -41,7 +41,7 @@ S = "${WORKDIR}/${BPN}${PV}"
 
 EXTRA_AUTORECONF += "--exclude=aclocal"
 
-CFLAGS += "-std=gnu17"
+CFLAGS:append = " -std=gnu17"
 
 do_install:append() {
     mv ${D}${libdir}/expect${PV}/libexpect*.so ${D}${libdir}/