Mark Ninja version with a `.fuchsia` prefix.

This will make it easier to verify that someone is using
the Fuchsia version of Ninja, instead of the upstream one.

Fuchsia-Only: Development helper.
Change-Id: Ia768de7a6f3f5add2431194899f13dcc686bb1a0
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/ninja-build/ninja/+/1025395
Reviewed-by: David Fang <fangism@google.com>
Fuchsia-Auto-Submit: David Turner <digit@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/src/version.cc b/src/version.cc
index d306957..9a47208 100644
--- a/src/version.cc
+++ b/src/version.cc
@@ -20,7 +20,7 @@
 
 using namespace std;
 
-const char* kNinjaVersion = "1.12.0.git";
+const char* kNinjaVersion = "1.12.0.git.fuchsia";
 
 void ParseVersion(const string& version, int* major, int* minor) {
   size_t end = version.find('.');