[refactor] Remove compat.h from kernel headers

This commit makes compat.h no longer a required include for users of the
libbitcoinkernel. Including compat.h imports a bunch of
platform-specific definitions.

This commit is part of the libbitcoinkernel project, namely its stage 1
step 3: Decouple most non-consensus headers from libbitcoinkernel.

src/random.cpp

@@ -5,6 +5,7 @@
 
 #include <random.h>
 
+#include <compat/compat.h>
 #include <compat/cpuid.h>
 #include <crypto/chacha20.h>
 #include <crypto/sha256.h>