selftests: vDSO: add --cflags for pkg-config command querying libsodium

When libsodium is installed into its own prefix, the --cflags output is
needed for the compiler to find libsodium headers.

Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>