scripts/dtc: Call pkg-config POSIXly correct

Running with POSIXLY_CORRECT=1 in the environment the scripts/dtc build
fails, because pkg-config doesn't output anything when the flags come
after the arguments.

Fixes: 067c650c ("dtc: Use pkg-config to locate libyaml")
Signed-off-by: Thomas Bracht Laumann Jespersen <t@laumann.xyz>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220131112028.7907-1-t@laumann.xyz