plugin-api: Fix the prefix for pkg_check_modules of CMakeLists.txt

plugin-api: Fix the prefix for pkg_check_modules of CMakeLists.txt

The command pkg_check_modules in CMakeLists.txt checks modules and sets
variables about these modules.
The names of these variables have their prefix designated by the command
pkg_check_modules like below:
	pkg_check_modules(<prefix> REQUIRED <modules>)

Previously, in plugin-api-common, prefix was "pkgs", which is not a
unique name.
In plugin-api-resourced, prefix was ${PROJECT_NAME} but when calling
variables, "pkgs" was used for prefix, which is not correct usage.
This fault was fixed by replacing their prefix as their ${PROJECT_NAME}.

Change-Id: Ide1db4b707f2c6e9f632c0cc36ab4ec654383887
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>