um: Fix -Wmissing-prototypes warnings for __warp_* and foo

These functions are not called explicitly. Let's just workaround
the -Wmissing-prototypes warnings by declaring them locally similar
to what was done in arch/x86/kernel/asm-offsets_32.c.

This will address below -Wmissing-prototypes warnings:

./arch/x86/um/shared/sysdep/kernel-offsets.h:9:6: warning: no previous prototype for ‘foo’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:187:7: warning: no previous prototype for ‘__wrap_malloc’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:208:7: warning: no previous prototype for ‘__wrap_calloc’ [-Wmissing-prototypes]
arch/um/os-Linux/main.c:222:6: warning: no previous prototype for ‘__wrap_free’ [-Wmissing-prototypes]
arch/x86/um/user-offsets.c:17:6: warning: no previous prototype for ‘foo’ [-Wmissing-prototypes]

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/main.c

@@ -184,6 +184,11 @@ int __init main(int argc, char **argv, char **envp)
 
 extern void *__real_malloc(int);
 
+/* workaround for -Wmissing-prototypes warnings */
+void *__wrap_malloc(int size);
+void *__wrap_calloc(int n, int size);
+void __wrap_free(void *ptr);
+
 void *__wrap_malloc(int size)
 {
 	void *ret;

arch/x86/um/shared/sysdep/kernel-offsets.h

@@ -6,6 +6,9 @@
 #include <linux/kbuild.h>
 #include <asm/mman.h>
 
+/* workaround for a warning with -Wmissing-prototypes */
+void foo(void);
+
 void foo(void)
 {
 #include <common-offsets.h>

arch/x86/um/user-offsets.c

@@ -14,6 +14,9 @@
 	COMMENT(#val " / sizeof(unsigned long)");	\
 	DEFINE(sym, val / sizeof(unsigned long))
 
+/* workaround for a warning with -Wmissing-prototypes */
+void foo(void);
+
 void foo(void)
 {
 #ifdef __i386__