Replace top-level `elif` with `if` in BUCK files.

Reviewed By: mzlee

Differential Revision: D4550165

fbshipit-source-id: 65d636c2e9e273bf45e093fe2c07aec18659e733

ReactCommon/cxxreact/BUCK

@@ -60,7 +60,7 @@ if THIS_IS_FBANDROID:
       )
     )
 
-elif THIS_IS_FBOBJC:
+if THIS_IS_FBOBJC:
   def react_library(**kwargs):
     ios_library(
       name = 'bridge',

ReactCommon/jschelpers/BUCK

@@ -31,7 +31,7 @@ if THIS_IS_FBANDROID:
     ],
   )
 
-elif THIS_IS_FBOBJC:
+if THIS_IS_FBOBJC:
   ios_library(
     name = 'jschelpers',
     inherited_buck_flags = STATIC_LIBRARY_IOS_FLAGS,