blocks: add byte option to add_const_v in GRC

The previous GRC yml description did not contain an entry for operation on
bytes.

Closes gnuradio#2461, Closes gnuradio#2354

Author: noc0lour

gr-blocks/grc/blocks_add_const_vxx.block.yml

@@ -6,11 +6,11 @@ parameters:
 -   id: type
     label: IO Type
     dtype: enum
-    options: [complex, float, int, short]
+    options: [complex, float, int, short, byte]
     option_attributes:
-        vconst_type: [complex_vector, real_vector, int_vector, int_vector]
-        const_type:  [complex, real, int, int]
-        fcn: [cc, ff, ii, ss]
+        vconst_type: [complex_vector, real_vector, int_vector, int_vector, int_vector]
+        const_type:  [complex, real, int, int, int]
+        fcn: [cc, ff, ii, ss, bb]
     hide: part
 -   id: const
     label: Constant