cni: clean dir ${B}/plugins before do_compile

Clean dir ${B}/plugins before do_compile to avoid cni generated binaries
like /usr/libexec/cni/bridge has wrong dynamic linker path and reports
error like: /usr/libexec/cni/bridge: no such file or directory".

Reproduce steps:
1. bitbake cni
2. enable usrmerge feature in local.conf
3. bitbake cni

After step 2, GOBUILDFLAGS changed,
"-I /lib64/ld-linux-aarch64.so.1" -> "/usr/lib/ld-linux-aarch64.so.1"

But "go build" seems only check if the cached packagefile changed, since
all not changed, the dynamic linker still use the old one, maybe go
build should improve this.

Clean dir ${B}/plugins to trigger rebuild of the binaries here.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

recipes-networking/cni/cni_git.bb

@@ -53,6 +53,8 @@ do_compile() {
 	done
 }
 
+do_compile[cleandirs] = "${B}/plugins"
+
 do_install() {
     localbindir="${libexecdir}/cni/"