Kernel: Avoid linking errors when calling Kernel API

The compiler can't see that the definitions inside the .h file aren't meant to be
public symbols. So in a hypothetical program which uses the Kernel API, each(\!)
compilation unit that includes FB.h would define those fb_get_size_in_bytes symbols.
If that happens twice or more times, that would cause linker errors.

Since the functions are very short, inlining them seems like a good idea.

Also, using FB.h should be possible even if the containing compilation unit
doesn't already define size_t, so I added that header (stddef), too.

Kernel/API/FB.h

@@ -26,32 +26,34 @@
 
 #pragma once
 
+#include <AK/Platform.h>
+#include <stddef.h>
 #include <sys/cdefs.h>
 #include <sys/ioctl.h>
 
 __BEGIN_DECLS
 
-int fb_get_size_in_bytes(int fd, size_t* out)
+ALWAYS_INLINE int fb_get_size_in_bytes(int fd, size_t* out)
 {
     return ioctl(fd, FB_IOCTL_GET_SIZE_IN_BYTES, out);
 }
 
-int fb_get_resolution(int fd, FBResolution* info)
+ALWAYS_INLINE int fb_get_resolution(int fd, FBResolution* info)
 {
     return ioctl(fd, FB_IOCTL_GET_RESOLUTION, info);
 }
 
-int fb_set_resolution(int fd, FBResolution* info)
+ALWAYS_INLINE int fb_set_resolution(int fd, FBResolution* info)
 {
     return ioctl(fd, FB_IOCTL_SET_RESOLUTION, info);
 }
 
-int fb_get_buffer(int fd, int* index)
+ALWAYS_INLINE int fb_get_buffer(int fd, int* index)
 {
     return ioctl(fd, FB_IOCTL_GET_BUFFER, index);
 }
 
-int fb_set_buffer(int fd, int index)
+ALWAYS_INLINE int fb_set_buffer(int fd, int index)
 {
     return ioctl(fd, FB_IOCTL_SET_BUFFER, index);
 }