riscv: add missing stdint.h include

While this is currently pulled in transitively, uint32_t is defined there
and we should explicitly include the appropriate header.

arch/riscv/include/arch/riscv/feature.h

@@ -9,6 +9,7 @@
 
 #include <lk/compiler.h>
 #include <stdbool.h>
+#include <stdint.h>
 #include <stdlib.h>
 
 __BEGIN_CDECLS