Avoid including <sys/syscall.h> on targets we already know about.

When cross-compiling to i686-unknown-linux-gnu from
x86_64-unknown-linux-gnu I found that this header wasn't found and the
build broken. Avoid that happening by only including this header when
we don't already know what it will tell us.

crypto/crypto.c

@@ -18,10 +18,6 @@
 
 #include <GFp/cpu.h>
 
-#if defined(__linux__)
-#include <sys/syscall.h>
-#endif
-
 #if defined(OPENSSL_WINDOWS)
 
 #if defined(_MSC_VER)
@@ -81,10 +77,14 @@ HIDDEN uint32_t GFp_armcap_P = 0;
 #elif defined(OPENSSL_X86_64)
 #define SYS_getrandom 318
 #else
+#include <sys/syscall.h>
+#if !defined(SYS_getrandom)
 #error "Error: Kernel headers are too old; SYS_getrandom not defined."
 #endif
 #endif
 
+#endif
+
 const long GFp_SYS_GETRANDOM = SYS_getrandom;
 #endif