C++20: don't use auto with u8"".

`u8""` has type `const char8_t*` rather than `const char*` in C++20, but
there's no `strlen()` for `char8_t*`, which breaks the build here.

Luckily this is the only place in the entire tree that does this. Let's
just remove the `auto`s.

Test: treehugger
Change-Id: I74c29889b4a60fdb642059c8699335c2abe99b34
diff --git a/liblog/tests/logprint_test.cpp b/liblog/tests/logprint_test.cpp
index 00be823..9d599ca 100644
--- a/liblog/tests/logprint_test.cpp
+++ b/liblog/tests/logprint_test.cpp
@@ -60,8 +60,8 @@
 TEST(liblog, convertPrintable_validutf8) {
   setlocale(LC_ALL, "C.UTF-8");
 
-  auto input = u8"¢ह€𐍈";
-  auto output_size = convertPrintable(nullptr, input, strlen(input));
+  const char* input = u8"¢ह€𐍈";
+  size_t output_size = convertPrintable(nullptr, input, strlen(input));
   EXPECT_EQ(output_size, strlen(input));
 
   char output[output_size + 1];