CMakeLists.txt: make it possible to build shared library

In cmake manner, building shared library is easily doable by specifying
BUILD_SHARED_LIBS option (with `-D` command line argument),
but this explicit `STATIC` specification prevents that.

Since STATIC build is the default, it can be simply removed.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0b4f1d4..25cc0e6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,7 +22,7 @@
 		src/common/Trace.cpp
         )
 
-add_library(oboe STATIC ${oboe_sources})
+add_library(oboe ${oboe_sources})
 
 # Specify directories which the compiler should look for headers
 target_include_directories(oboe