Theoretical Fix issue with BoringSSL-GRPC via update to the podspec

[ulmentflam]

Fix to issue #342

Taking note from how Firebase uses preserve_path https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseFirestore.podspec#L54 and the issue created on the CocoaPods repo CocoaPods/CocoaPods#10772.

This appears to fix my issue and allows my applications to build and the LibXlsxWriterSwiftSample to build with 'BoringSSL-GRPC'.

The LibXlsxWriterSwiftSample uses the following pod file to run with BoringSSL-GRPC:

# Uncomment the next line to define a global platform for your project
platform :ios, '13.0'

target 'LibXlsxWriterSwiftSample' do
  # Comment the next line if you don't want to use dynamic frameworks
  use_frameworks!

  # Pods for LibXlsxWriterSwiftSample
  pod 'libxlsxwriter', :git => 'https://github.com/52inc/libxlsxwriter.git'
  pod 'Sourceful', '~> 0.2' #Only present for synzax highlighting
  pod 'BoringSSL-GRPC'
end

[jmcnamara]

Merged manually. Thanks.