Refine linkage for inline functions in drm.h (#3898)

Removed static linkage on inline functions SbDrmSystemIsValid() and
SbDrmTicketIsValid().

Marking function as inline is enough to permit multiple definitions of
the function, and adding static will cause warnings in compile units
where the function is unused.

b/276483058

starboard/drm.h

@@ -225,12 +225,12 @@ typedef void (*SbDrmServerCertificateUpdatedFunc)(SbDrmSystem drm_system,
 // --- Functions -------------------------------------------------------------
 
 // Indicates whether |drm_system| is a valid SbDrmSystem.
-static SB_C_FORCE_INLINE bool SbDrmSystemIsValid(SbDrmSystem drm) {
+SB_C_FORCE_INLINE bool SbDrmSystemIsValid(SbDrmSystem drm) {
   return drm != kSbDrmSystemInvalid;
 }
 
 // Indicates whether |ticket| is a valid ticket.
-static SB_C_FORCE_INLINE bool SbDrmTicketIsValid(int ticket) {
+SB_C_FORCE_INLINE bool SbDrmTicketIsValid(int ticket) {
   return ticket != kSbDrmTicketInvalid;
 }