firmware/layer1: avoid 'for' loop initial declarations

As was reported by roox, osmocom-bb currently fails to build on OBS:

https://build.opensuse.org/build/home:mnhauke:osmocom:nightly/openSUSE_Tumbleweed/x86_64/osmocom-bb/_log

[   24s] layer1/prim_tch.c: In function 'l1s_tch_meas_avg':
[   24s] layer1/prim_tch.c:183:2: error: 'for' loop initial declarations are only allowed in C99 mode
[   24s] layer1/prim_tch.c:183:2: note: use option -std=c99 or -std=gnu99 to compile your code

We don't specify the C standard explicitly, so let's move the variable
declaration out of the for-loop in l1s_tch_meas_avg().

Change-Id: I6c65fbead4e612c81728e9c6601d5f2107616ee6
Fixes: 7286560a3 "firmware/layer1: fill-in DL info for L1CTL TRAFFIC.ind"
---
M src/target/firmware/layer1/prim_tch.c
1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/target/firmware/layer1/prim_tch.c b/src/target/firmware/layer1/prim_tch.c
index a33cb58..1bc842b 100644
--- a/src/target/firmware/layer1/prim_tch.c
+++ b/src/target/firmware/layer1/prim_tch.c
@@ -179,8 +179,9 @@
 {
 	uint32_t avg_snr = 0;
 	int32_t avg_dbm8 = 0;
+	unsigned int i;
 
-	for (unsigned int i = 0; i < meas_num; i++) {
+	for (i = 0; i < meas_num; i++) {
 		int j = (rx_tch.meas_id + FACCH_MEAS_HIST - i) % FACCH_MEAS_HIST;
 		avg_snr += rx_tch.meas[j].snr;
 		avg_dbm8 += rx_tch.meas[j].pm_dbm8;

