qmic: Add missing string type in structs

Structs can contain strings, we still don't encode and decode these but
this change makes the generated header file compilable.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

qmi_struct.c

@@ -7,6 +7,7 @@ static const char *sz_simple_types[] = {
 	[TYPE_U16] = "uint16_t",
 	[TYPE_U32] = "uint32_t",
 	[TYPE_U64] = "uint64_t",
+	[TYPE_STRING] = "char *",
 };
 
 struct qmi_struct_member {