cc-mode/if: escape backslashes

Escaping lets the user choose between writing an unbraced `if` by
pressing C-d, or continue with a braced `if` by pressing TAB. The
current behavior when pressing C-d at this point is that there is an
unbalanced right brace remaining.

snippets/cc-mode/if

@@ -2,6 +2,6 @@
 # name: if (...) { ... }
 # key: if
 # --
-if (${1:condition}) ${2:{
+if (${1:condition}) ${2:\{
     $0
-}}
+\}}