x86/fpu: Include asm/fpu/regset.h

The fpregs_soft_set/fpregs_soft_get functions are declared in a
header that is not included in the file that defines them, causing
a W=1 warning:

/home/arnd/arm-soc/arch/x86/math-emu/fpu_entry.c:638:5: error: no previous prototype for 'fpregs_soft_set' [-Werror=missing-prototypes]
  638 | int fpregs_soft_set(struct task_struct *target,
      |     ^~~~~~~~~~~~~~~
/home/arnd/arm-soc/arch/x86/math-emu/fpu_entry.c:690:5: error: no previous prototype for 'fpregs_soft_get' [-Werror=missing-prototypes]
  690 | int fpregs_soft_get(struct task_struct *target,

Include the file here to avoid the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://lore.kernel.org/all/20230516193549.544673-7-arnd%40kernel.org

arch/x86/math-emu/fpu_entry.c

@@ -32,6 +32,7 @@
 #include <asm/traps.h>
 #include <asm/user.h>
 #include <asm/fpu/api.h>
+#include <asm/fpu/regset.h>
 
 #include "fpu_system.h"
 #include "fpu_emu.h"