llbuild: guard the complete header from duplicate inclusion

The header guards prevent re-processing the header when re-referenced.
We were not guarding the entire file. Although this matters very little
in practice in this library as the headers will not change behaviour and
inclusion into Swift will go through a module, this is a small drive-by
clean up that makes code more idiomatic.

Author: compnerd

products/libllbuild/include/llbuild/llbuild.h

@@ -30,7 +30,6 @@ LLBUILD_EXPORT int llb_get_api_version(void);
 
 // The BuildSystem component.
 #include "buildsystem.h"
-#endif
 
 // The Database component.
 #include "db.h"
@@ -39,3 +38,5 @@ LLBUILD_EXPORT int llb_get_api_version(void);
 #include "buildvalue.h"
 
 #include "ninja.h"
+
+#endif