libxmlrpc: add missing braces for variable CURL_CONFIG in patch 4

Change $CURL_CONFIG to $(CURL_CONFIG) in the patch
0004-use-correct-curl-config.patch because in a makefile the braces are
required to expand a variable.

Fix error messages:
make[3]: Entering directory '/home/tetsuya/buildroot/br2/output/build/libxmlrpc-1.25.30/lib/curl_transport'
make[3]: URL_CONFIG: Command not found
/bin/sh: line 0: test: <: unary operator expected
make[3]: URL_CONFIG: Command not found

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>