Fix headers installation path

Filed upstream at https://bugs.launchpad.net/maria/+bug/886550


diff -Naur 5.5_orig/include/CMakeLists.txt 5.5/include/CMakeLists.txt
--- 5.5_orig/include/CMakeLists.txt	2011-11-01 16:20:49.000000000 +0100
+++ 5.5/include/CMakeLists.txt	2011-11-05 14:21:33.924984384 +0100
@@ -60,4 +60,4 @@
 )
 
 INSTALL(FILES ${HEADERS} DESTINATION ${INSTALL_INCLUDEDIR} COMPONENT Development)
-INSTALL(DIRECTORY mysql/ DESTINATION ${INSTALL_INCLUDEDIR}/mysql COMPONENT Development FILES_MATCHING PATTERN "*.h")
+INSTALL(DIRECTORY mysql/ DESTINATION ${INSTALL_INCLUDEDIR} COMPONENT Development FILES_MATCHING PATTERN "*.h")