riscv64: missing EFI_FUNCTION

Add missing definition. It is needed for 'make apps'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

inc/riscv64/efibind.h

@@ -29,3 +29,5 @@ typedef uint16_t WCHAR;
 #define EFI_SIGNATURE_16(A,B)             ((A) | (B<<8))
 #define EFI_SIGNATURE_32(A,B,C,D)         (EFI_SIGNATURE_16(A,B)     | (EFI_SIGNATURE_16(C,D)     << 16))
 #define EFI_SIGNATURE_64(A,B,C,D,E,F,G,H) (EFI_SIGNATURE_32(A,B,C,D) | ((UINT64)(EFI_SIGNATURE_32(E,F,G,H)) << 32))
+
+#define EFI_FUNCTION