app/test: fix resource creation with objcopy on FreeBSD

app/test: fix resource creation with objcopy on FreeBSD

Using of the /dev/stdin generates a warning when compiling on FreeBSD:

 objcopy: Warning: '/dev/stdin' is not an ordinary file
 app/test/Makefile:78: recipe for target 'test_resource_c.res.o' failed
 # ls -l /dev/stdin
 lrwxr-xr-x  1 root  wheel  4 Jun 17 12:24 /dev/stdin -> fd/0

Replace /dev/stdin by a temporary file.

Reported-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Tested-by: Thomas Monjalon <thomas.monjalon@6wind.com>