contrib/elf2dmp: Build download.o with CURL_CFLAGS

contrib/elf2dmp has a source file which uses curl/curl.h;
although we link the final executable with CURL_LIBS, we
forgot to build this source file with CURL_CFLAGS, so if
the curl header is in a place that's not already on the
system include path then it will fail to build.

Add a line specifying the cflags needed for download.o;
while we are here, bring the specification of the libs
into line with this, since using a per-object variable
setting is preferred over adding them to the final
executable link line.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190719100955.17180-1-peter.maydell@linaro.org