[tools] Add back an accidentally-disabled linter config (RobotLocomot…

…ion#22274)

CPPLINT.cfg

@@ -20,6 +20,12 @@ filter=+build/pragma_once
 # Disable cpplint's include order.  We have our own via //tools:drakelint.
 filter=-build/include_order
 
+# Filters are prefix strings on the name of the error category (i.e., string
+# matches are anchored only at the start of the name, not the end), so we must
+# explicitly opt-in to longer category names that the `-build/include` filter in
+# the next paragraph would otherwise disable.
+filter=+build/include_what_you_use
+
 # Disable cpplint's check to include the h file atop the cc file (along with
 # some other, even less relevant checks). Given the layout of bazel's runfiles
 # during linting and our include path customizations, cpplint yields false

math/matrix_util.h

@@ -4,6 +4,7 @@
 #include <cmath>
 #include <functional>
 #include <set>
+#include <string>
 #include <string_view>
 #include <utility>
 #include <vector>