format: Add support for classic .gsm file/frame format

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/src/formats.c b/src/formats.c
index 1f54c05..5c91a52 100644
--- a/src/formats.c
+++ b/src/formats.c
@@ -22,8 +22,11 @@
 #include <gapk/formats.h>
 
 /* Extern format descriptors */
+extern const struct format_desc fmt_gsm;
+
 static const struct format_desc *supported_formats[_FMT_MAX] = {
 	[FMT_INVALID]		= NULL,
+	[FMT_GSM]		= &fmt_gsm,
 };