[MIPS] Fix missing prototypes in asm/fpu.h

While building the Malta defconfig, sparse spat the following
warnings:

>>>>>>>>>>>>>>>>>>
arch/mips/math-emu/kernel_linkage.c:31:6: warning: symbol
'fpu_emulator_init_fpu' was not declared. Should it be static?

arch/mips/math-emu/kernel_linkage.c:54:5: warning: symbol
'fpu_emulator_save_context' was not declared. Should it be
static?

arch/mips/math-emu/kernel_linkage.c:68:5: warning: symbol
'fpu_emulator_restore_context' was not declared. Should it be
static?
>>>>>>>>>>>>>>>>>>

This patch fixes these errors by adding the proper prototypes
to the include/asm-mips/fpu.h header, and actually using this
header in the sparse-spotted source file.

Build-tested with Malta defconfig.

Signed-off-by: Dmitri Vorobiev <[e-mail]>
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/math-emu/kernel_linkage.c

···
 #include <asm/signal.h>
 #include <asm/uaccess.h>
 
+#include <asm/fpu.h>
 #include <asm/fpu_emulator.h>
 
 #define SIGNALLING_NAN 0x7ff800007ff80000LL

include/asm-mips/fpu.h

···
 extern asmlinkage int (*restore_fp_context32)(struct sigcontext32 __user *sc);
 
 extern void fpu_emulator_init_fpu(void);
+extern int fpu_emulator_save_context(struct sigcontext __user *sc);
+extern int fpu_emulator_restore_context(struct sigcontext __user *sc);
 extern void _init_fpu(void);
 extern void _save_fp(struct task_struct *);
 extern void _restore_fp(struct task_struct *);