README: Run ldconfig with sudo (native install)

ldconfig requires access to /etc/ld.so.cache which is owned by root. Let
it run with sudo to access the ld.so.cache file and the shared
libraries so that linking works correctly.

Change-Id: I52df5a0b70c1975634974a31eb76aa45becec161

README.md

@@ -50,7 +50,7 @@ mkdir build
 cd build
 cmake -DCMAKE_BUILD_TYPE=Release ..
 sudo make && sudo make install
-ldconfig
+sudo ldconfig
 
 git clone https://github.com/pierotofy/PotreeConverter /staging/PotreeConverter
 cd /staging/PotreeConverter