cmake: fix warning when searching for SKIQ

Replace INCLUDE(FindPkgConfig) by FIND_PACKAGE(PkgConfig REQUIRED)
to avoid following warning:

warning: The package name passed to `find_package_handle_standard_args` (PkgConfig) does not match the name of the calling package (SKIQ).  This can lead to problems in calling code that expects `find_package` result variables (e.g., `_FOUND`) to follow a certain pattern.

cmake/modules/FindSKIQ.cmake

@@ -6,7 +6,7 @@
 # the distribution.
 #
 
-INCLUDE(FindPkgConfig)
+FIND_PACKAGE(PkgConfig REQUIRED)
 #PKG_CHECK_MODULES(SKIQ SKIQ)
 IF(NOT SKIQ_FOUND)