Fix travis build.

Regression was introduced in 5faca8d4f6.

GYP patch for GCC precompiled header support was accidentally removed.

docs/building-cmake.md

@@ -180,6 +180,7 @@ In Terminal go to **/home/user/TBuild/Libraries** and run
     wget https://cmake.org/files/v3.6/cmake-3.6.2.tar.gz
     tar -xf cmake-3.6.2.tar.gz
     cd gyp
+    git checkout 702ac58e47
     git apply ../../tdesktop/Telegram/Patches/gyp.diff
     cd ../cmake-3.6.2
     ./configure

docs/building-xcode.md

@@ -194,6 +194,8 @@ In Terminal go to **/Users/user/TBuild/Libraries** and run:
     git clone https://chromium.googlesource.com/external/gyp
     git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
     cd gyp
+    git checkout 702ac58e47
+    git apply ../../tdesktop/Telegram/Patches/gyp.diff
     ./setup.py build
     sudo ./setup.py install
     cd ..