[fuchsia] Link against zxcpp

Previously, we were linking boringssl dynamically against libc++.so.
Because we include boringssl in the SDK, we want to not link against
libc++.so.

In order for the C++ language to work, we need to link against some
library that provides a few basic symbols. This CL obtains those symbols
by statically linking against zxcpp, which is how we normally get those
symbols without libc++.

Test: SDK compiles
Change-Id: I43e4220a62ea1baae1a172f21b781a496842db44
diff --git a/BUILD.gn b/BUILD.gn
index 3bc6b34..ac61081 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -67,6 +67,7 @@
   }
   if (is_fuchsia) {
     include_base = "//third_party/boringssl/src/include"
+    deps = [ "//zircon/public/lib/zxcpp" ]
   }
 }
 
@@ -84,6 +85,7 @@
   if (is_fuchsia) {
     runtime_deps = [ ":crypto_sdk" ]
     include_base = "//third_party/boringssl/src/include"
+    deps += [ "//zircon/public/lib/zxcpp" ]
   }
 }