system/audio.h: Make function inline

This patch makes the audio_device_address_to_parameter inline. All of the other
helpers defined in this header are inline functions except for this one and this
causes compile failures when -Werror=unused-function is used.

BUG=21339022

Change-Id: I7db58bf7bed3b89b6752ec2cd2c1f358ba2a4d89
diff --git a/include/system/audio.h b/include/system/audio.h
index 8d9ec88..ced2005 100644
--- a/include/system/audio.h
+++ b/include/system/audio.h
@@ -1354,7 +1354,7 @@
 }
 
 /* converts device address to string sent to audio HAL via set_parameters */
-static char *audio_device_address_to_parameter(audio_devices_t device, const char *address)
+static inline char *audio_device_address_to_parameter(audio_devices_t device, const char *address)
 {
     const size_t kSize = AUDIO_DEVICE_MAX_ADDRESS_LEN + sizeof("a2dp_sink_address=");
     char param[kSize];