Fix examples linking with CMake and -DPLATFORM=SDL

Currently, every example fails linking likeso:

[  3%] Linking C executable audio_mixed_processor
/usr/bin/ld: ../raylib/libraylib.a(raudio.c.o): undefined reference to symbol 'exp@@GLIBC_2.29'
/usr/bin/ld: /usr/lib/libm.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Apparently, linking libm explicitly is the solution.

examples/CMakeLists.txt

@@ -130,7 +130,7 @@ foreach (example_source ${example_sources})
     # Setup the example
     add_executable(${example_name} ${example_source})
 
-    target_link_libraries(${example_name} raylib)
+    target_link_libraries(${example_name} raylib m)
 
     string(REGEX MATCH ".*/.*/" resources_dir ${example_source})
     string(APPEND resources_dir "resources")