When running the preprocessor to determine whether pthread is enabled, only
use TF-PSA-Crypto include paths. Don't use the rest of `LOCAL_CFLAGS`,
including Mbed TLS include paths, which aren't really useful here.
This will simplify later refactorings, because it simplifies a dependency
chain [crypto paths] → `LOCAL_CFLAGS` → `THREADING` → `LOCAL_LDFLAGS`
into just [crypto paths] → `THREADING` → `LOCAL_LDFLAGS`.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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