Pin BoringSSL to a known working revision to prevent build failures

As reported by Orlando pin the version of the BoringSSL library to
prevent build failures.

Signed-off-by: Sebastian Ene <sebastianene@google.com>
diff --git a/default.xml b/default.xml
index db09f3c..e1f103c 100644
--- a/default.xml
+++ b/default.xml
@@ -51,7 +51,7 @@
 
   <!-- BEGIN platform projects -->
   <project path="development" name="platform/development" groups="trusty" />
-  <project path="external/boringssl" name="platform/external/boringssl" groups="trusty" />
+  <project path="external/boringssl" name="platform/external/boringssl" groups="trusty" revision="7cca763839270d06b1dc1726753c0e0eec003b50" />
   <project path="external/fmtlib" name="platform/external/fmtlib" groups="trusty" />
   <project path="external/freetype" name="platform/external/freetype" groups="trusty" />
   <project path="external/googletest" name="platform/external/googletest" groups="trusty" />