vppinfra: explicitely state for signed types that they are signed

This fixes some compilation warnings with clang on AArch64.

Change-Id: Idb941944e3f199f483c80e143a9e5163a031c4aa
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/src/vppinfra/macros.c b/src/vppinfra/macros.c
index ce4cc9b..6649277 100644
--- a/src/vppinfra/macros.c
+++ b/src/vppinfra/macros.c
@@ -144,7 +144,7 @@
 	      /* Look in value table */
 	      if (!varvalue)
 		{
-		  char *tmp = clib_macro_get_value (mm, varname);
+		  i8 *tmp = clib_macro_get_value (mm, (char *) varname);
 		  if (tmp)
 		    varvalue = (i8 *) format (0, "%s%c", tmp, 0);
 		}
@@ -152,7 +152,7 @@
 	      /* Look in environment. */
 	      if (!varvalue)
 		{
-		  char *tmp = getenv (varname);
+		  char *tmp = getenv ((char *) varname);
 		  if (tmp)
 		    varvalue = (i8 *) format (0, "%s%c", tmp, 0);
 		}