CLEANUP: sample: uninline sample_conv_var2smp_str()

There's no reason to limit this one to this file, it could be used in
other contexts.
diff --git a/src/sample.c b/src/sample.c
index 64a4b57..67853d2 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -1744,7 +1744,7 @@
  *
  * This function returns 0 if an error occurs, otherwise it returns 1.
  */
-static inline int sample_conv_var2smp_str(const struct arg *arg, struct sample *smp)
+int sample_conv_var2smp_str(const struct arg *arg, struct sample *smp)
 {
 	switch (arg->type) {
 	case ARGT_STR: