Add missing string.h include in //base/test

The code uses strlen in generate_fontconfig_caches.cc, but does not
include string.h. This fails on Linux when building the code against
glibc newer than 2.29.

Bug: 1074286
Change-Id: I1c0d00a2a4b312b770882cf992007b5e80b1978d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2164527Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762422}

base/test/generate_fontconfig_caches.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include <fontconfig/fontconfig.h>
+#include <string.h>
 #include <time.h>
 #include <utime.h>