build/flags-if: Fix sed expression on BSD

`p` needs a terminating semicolon; otherwise we get

    sed: 1: "\|^///|{s|^/// ||; s|[^ ...": extra characters at the end of p command

Fixes: 3552b79 ("build/flags: Infrastructure to detect compiler flag support")

build/flags-if.sh

@@ -8,7 +8,7 @@
 set -eu
 
 OLD_FLAGS="$XCC $XCPPFLAGS $XCFLAGS $XLDFLAGS $XLDLIBS"
-NEW_FLAGS=$(sed -n '\|^///|{s|^/// ||; s|[^=]*= ||; p}' "$1")
+NEW_FLAGS=$(sed -n '\|^///|{s|^/// ||; s|[^=]*= ||; p;}' "$1")
 build/cc.sh "$@" $NEW_FLAGS || exit 1
 
 # De-duplicate against the existing flags