Kernel: Mark `serenity_dev_` functions as static

This avoids multiple definition errors when linking software which
may utilize these functions from different compilation units.

Kernel/API/Device.h

@@ -12,17 +12,17 @@
 
 __BEGIN_DECLS
 
-ALWAYS_INLINE dev_t serenity_dev_makedev(unsigned major, unsigned minor)
+static ALWAYS_INLINE dev_t serenity_dev_makedev(unsigned major, unsigned minor)
 {
     return (minor & 0xffu) | (major << 8u) | ((minor & ~0xffu) << 12u);
 }
 
-ALWAYS_INLINE unsigned int serenity_dev_major(dev_t dev)
+static ALWAYS_INLINE unsigned int serenity_dev_major(dev_t dev)
 {
     return (dev & 0xfff00u) >> 8u;
 }
 
-ALWAYS_INLINE unsigned int serenity_dev_minor(dev_t dev)
+static ALWAYS_INLINE unsigned int serenity_dev_minor(dev_t dev)
 {
     return (dev & 0xffu) | ((dev >> 12u) & 0xfff00u);
 }