upkeep: Use char overload of find_last_of where possible

Change-Id: Iece01e12059e720e7c9a7cb75e870f145e4c2a17
diff --git a/src/base/utils.cc b/src/base/utils.cc
index 041e010..9b145fb 100644
--- a/src/base/utils.cc
+++ b/src/base/utils.cc
@@ -181,9 +181,9 @@
   auto path = GetCurExecutablePath();
 #if PERFETTO_BUILDFLAG(PERFETTO_OS_WIN)
   // Paths in Windows can have both kinds of slashes (mingw vs msvc).
-  path = path.substr(0, path.find_last_of("\\"));
+  path = path.substr(0, path.find_last_of('\\'));
 #endif
-  path = path.substr(0, path.find_last_of("/"));
+  path = path.substr(0, path.find_last_of('/'));
   return path;
 }