m68k: math-emu: Make multi_arith.h self-contained

Add the missing #include "fp_emu.h".
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/2efd6610e97dd1621e7d28bf87cb79173285545d.1692283195.git.geert@linux-m68k.org

arch/m68k/math-emu/multi_arith.h

@@ -18,6 +18,8 @@
 #ifndef _MULTI_ARITH_H
 #define _MULTI_ARITH_H
 
+#include "fp_emu.h"
+
 static inline void fp_denormalize(struct fp_ext *reg, unsigned int cnt)
 {
 	reg->exp += cnt;