libcurl: fix up prefix in curl-config

While most packages that rely on libcurl use pkg-config, a few
packages (such as libxmlrpc) still use curl-config. So we need to fix
up the prefix locations in the curl-config script installed in
STAGING_DIR.

[Peter: only fixup if prefix=/usr]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>