curlpp: fix override syntax

Commit bca3bbbf20 cherry-picked a fix from master, which used
the new override syntax, which was introduced in poky commit
2abf8a699edd513405be (2021-07-25, "bitbake: bitbake: Switch to using new
override syntax"). However, this change was merged after 3.4_M2 and is
not part of hardknott, so bitbake complains about the new syntax:

    ERROR: ParseError at
    …/meta-openembedded/meta-networking/recipes-support/curlpp/curlpp_0.8.1.bb:20:
    unparsed line: 'do_install:append() {'

Revert to the old syntax on the hardknott branch for now.

Fixes: bca3bbbf20 (2021-08-09, "curlpp: fix QA Issue after LDFLAGS change")
Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-networking/recipes-support/curlpp/curlpp_0.8.1.bb

@@ -17,7 +17,7 @@ inherit cmake pkgconfig binconfig
 
 BBCLASSEXTEND = "native nativesdk"
 
-do_install:append() {
+do_install_append() {
     sed -e 's@[^ ]*-ffile-prefix-map=[^ "]*@@g' \
         -e 's@[^ ]*-fdebug-prefix-map=[^ "]*@@g' \
         -e 's@[^ ]*-fmacro-prefix-map=[^ "]*@@g' \