Fix the Private path error in the podspec file.

Author: pepsikirk

SDWebImageWebPCoder.podspec

@@ -18,8 +18,8 @@ This is a SDWebImage coder plugin to support WebP image.
   s.watchos.deployment_target = '2.0'
   s.module_map = 'SDWebImageWebPCoder/Module/SDWebImageWebPCoder.modulemap'
 
-  s.source_files = 'SDWebImageWebPCoder/Classes/**/*', 'SDWebImageWebPCoder/Classes/Private/*.h', 'SDWebImageWebPCoder/Module/SDWebImageWebPCoder.h'
-  s.private_header_files = 'SDWebImageWebPCoder/Classes/Private/*.h'
+  s.source_files = 'SDWebImageWebPCoder/Classes/**/*', 'SDWebImageWebPCoder/Private/*.h', 'SDWebImageWebPCoder/Module/SDWebImageWebPCoder.h'
+  s.private_header_files = 'SDWebImageWebPCoder/Private/*.h'
   s.xcconfig = {
     'GCC_PREPROCESSOR_DEFINITIONS' => '$(inherited) SD_WEBP=1',
     'USER_HEADER_SEARCH_PATHS' => '$(inherited) $(SRCROOT)/libwebp/src'