conv: Fix main() of conv_test to compile with clang.

Fix type of argument 'argv'.

Addresses:
conv/conv_test.c:358:5: error: second parameter of 'main' (argument
array) must
      be of type 'char **'
int main(int argc, char argv[])
diff --git a/tests/conv/conv_test.c b/tests/conv/conv_test.c
index 54e043e..ab651d4 100644
--- a/tests/conv/conv_test.c
+++ b/tests/conv/conv_test.c
@@ -1,5 +1,6 @@
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <time.h>
 
 #include <osmocom/core/bits.h>
@@ -355,7 +356,7 @@
 }
 
 
-int main(int argc, char argv[])
+int main(int argc, char *argv[])
 {
 	const struct conv_test_vector *tst;
 	ubit_t *bu0, *bu1;