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>

tools/testing/selftests/vDSO/Makefile

 # SPDX-License-Identifier: GPL-2.0
 uname_M := $(shell uname -m 2>/dev/null || echo not)
 ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
-SODIUM := $(shell pkg-config --libs libsodium 2>/dev/null)
+SODIUM := $(shell pkg-config --libs --cflags libsodium 2>/dev/null)
 
 TEST_GEN_PROGS := vdso_test_gettimeofday
 TEST_GEN_PROGS += vdso_test_getcpu