tools/power/x86/intel-speed-select: Use pkg-config for libnl-3.0 detection

Replace hardcoded libnl3 include path with pkg-config detection to
improve portability across different distributions and build environments.

The previous implementation used a fixed path constructed from the
compiler's sysroot, which could fail on systems with non-standard
library installations. Now the build system:
- Attempts to detect libnl-3.0 include paths using pkg-config
- Falls back to /usr/include/libnl3 if pkg-config is unavailable
- Maintains backward compatibility with existing build configurations

This ensures the tool builds correctly on a wider range of systems
while preserving existing behavior when pkg-config is not present.

Closes:https://bugzilla.kernel.org/show_bug.cgi?id=220819
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>

tools/power/x86/intel-speed-select/Makefile

@@ -13,7 +13,13 @@ endif
 # Do not use make's built-in rules
 # (this improves performance and avoids hard-to-debug behaviour);
 MAKEFLAGS += -r
-override CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include -I$(shell $(CC) -print-sysroot)/usr/include/libnl3
+
+NL3_CFLAGS = $(shell pkg-config --cflags libnl-3.0 2>/dev/null)
+ifeq ($(NL3_CFLAGS),)
+NL3_CFLAGS = -I/usr/include/libnl3
+endif
+
+override CFLAGS += -O2 -Wall -g -D_GNU_SOURCE -I$(OUTPUT)include $(NL3_CFLAGS)
 override LDFLAGS += -lnl-genl-3 -lnl-3
 
 ALL_TARGETS := intel-speed-select