selftests: vDSO: separate LDLIBS from CFLAGS for libsodium

On systems that set -Wl,--as-needed, putting the -lsodium in the wrong
place on the command line means we get a linker error:

      CC       vdso_test_chacha
    /usr/bin/ld: /tmp/ccKpjnSM.o: in function `main':
    vdso_test_chacha.c:(.text+0x276): undefined reference to `crypto_stream_chacha20'
    collect2: error: ld returned 1 exit status

Fix this by passing pkg-config's --libs output to the LDFLAGS field
instead of the CFLAGS field, as is customary.

Reported-by: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>